# Bash-Oneliner
I am glad that you are here! I was working on bioinformatics a few years ago and was amazed by those single-word bash commands which are much faster than my dull scripts, time saved through learning command-line shortcuts and scripting. Recent years I am working on cloud computing and I keep recording those useful commands here. Not all of them is oneliner, but i put effort on making them brief and swift. I am mainly using Ubuntu, Amazon Linux, RedHat, Linux Mint, Mac and CentOS, sorry if the commands don't work on your system.

This blog will focus on simple bash commands for parsing data and Linux system maintenance that i acquired from work and LPIC exam. I apologize that there are no detailed citation for all the commands, but they are probably from dear search engine and Stack Overflow.

English and bash are not my first language, please correct me anytime, thank you.
If you know other cool commands, please teach me!

Here's a more stylish version of [Bash-Oneliner](https://onceupon.github.io/Bash-Oneliner/)~

## Handy Bash one-liners

- [Terminal Tricks](#terminal-tricks)
- [Variable](#variable)
- [Math](#math)
- [Grep](#grep)
- [Sed](#sed)
- [Awk](#awk)
- [Xargs](#xargs)
- [Find](#find)
- [Condition and Loop](#condition-and-loop)
- [Time](#time)
- [Download](#download)
- [Random](#random)
- [Xwindow](#xwindow)
- [System](#system)
- [Hardware](#hardware)
- [Networking](#networking)
- [Data Wrangling](#data-wrangling)
- [Others](#others)

## Terminal Tricks

#####  Using Ctrl keys
```
Ctrl + a : move to the beginning of line.
Ctrl + d : if you've type something, Ctrl + d deletes the character under the cursor, else, it escapes the current shell.
Ctrl + e : move to the end of line.
Ctrl + k : delete all text from the cursor to the end of line.
Ctrl + l : equivalent to clear.
Ctrl + n : same as Down arrow.
Ctrl + p : same as Up arrow.
Ctrl + q : to resume output to terminal after Ctrl + s.
Ctrl + r : begins a backward search through command history.(keep pressing Ctrl + r to move backward)
Ctrl + s : to stop output to terminal.
Ctrl + t : transpose the character before the cursor with the one under the cursor, press Esc + t to transposes the two words before the cursor.
Ctrl + u : cut the line before the cursor; then Ctrl + y paste it
Ctrl + w : cut the word before the cursor; then Ctrl + y paste it
Ctrl + x + backspace : delete all text from the beginning of line to the cursor.
Ctrl + x + Ctrl + e : launch editor defined by $EDITOR to input your command. Useful for multi-line commands.
Ctrl + z : stop current running process and keep it in background. You can use `fg` to continue the process in the foreground, or `bg` to continue the process in the background.
Ctrl + _ : undo typing.
```

##### Change case
```bash
Esc + u
# converts text from cursor to the end of the word to uppercase.
Esc + l
# converts text from cursor to the end of the word to lowercase.
Esc + c
# converts letter under the cursor to uppercase, rest of the word to lowercase.
```

##### Run history number (e.g. 53)
```bash
!53
```

##### Run last command
```bash
!!
# run the previous command using sudo
sudo !!
```

##### Run last command and change some parameter using caret substitution (e.g. last command: echo 'aaa' -> rerun as: echo 'bbb')
```bash
#last command: echo 'aaa'
^aaa^bbb

#echo 'bbb'
#bbb

#Notice that only the first aaa will be replaced, if you want to replace all 'aaa', use ':&' to repeat it:
^aaa^bbb^:&
#or
!!:gs/aaa/bbb/
```

##### Run past command that began with (e.g. cat filename)
```bash
!cat
# or
!c
# run cat filename again
```

##### Bash globbing
```bash
# '*' serves as a "wild card" for filename expansion.
/etc/pa*wd    #/etc/passwd

# '?' serves as a single-character "wild card" for filename expansion.
/b?n/?at      #/bin/cat

# '[]' serves to match the character from a range.
ls -l [a-z]*   #list all files with alphabet in its filename.

# '{}' can be used to match filenames with more than one patterns
ls *.{sh,py}   #list all .sh and .py files
```

##### Some handy environment variables
```
$0   :name of shell or shell script.
$1, $2, $3, ... :positional parameters.
$#   :number of positional parameters.
$?   :most recent foreground pipeline exit status.
$-   :current options set for the shell.
$$   :pid of the current shell (not subshell).
$!   :is the PID of the most recent background command.
$_   :last argument of the previously executed command, or the path of the bash script.

$DESKTOP_SESSION     current display manager
$EDITOR   preferred text editor.
$LANG   current language.
$PATH   list of directories to search for executable files (i.e. ready-to-run programs)
$PWD    current directory
$SHELL  current shell
$USER   current username
$HOSTNAME   current hostname
```

##### Using vi-mode in your shell
```bash
set -o vi
# change bash shell to vi mode
# then hit the Esc key to change to vi edit mode (when `set -o vi` is set)
k
# in vi edit mode - previous command
j
# in vi edit mode - next command
0
# in vi edit mode - beginning of the command
R
# in vi edit mode - replace current characters of command
2w
# in vi edit mode - next to 2nd word
b
# in vi edit mode - previous word
i
# in vi edit mode - go to insert mode
v
# in vi edit mode - edit current command in vi
man 3 readline
# man page for complete readline mapping
```

## Variable
[[back to top](#handy-bash-one-liners)]
##### Variable substitution within quotes
```bash
# foo=bar
echo $foo
# bar
echo "$foo"
# bar
# single quotes cause variables to not be expanded
echo '$foo'
# $foo
# single quotes within double quotes will not cancel expansion and will be part of the output
echo "'$foo'"
# 'bar'
# doubled single quotes act as if there are no quotes at all
echo ''$foo''
# bar
```

##### Get the length of variable
```bash
var="some string"
echo ${#var}
# 11
```

##### Get the first character of the variable
```bash
var=string
echo "${var:0:1}"
#s

# or
echo ${var%%"${var#?}"}
```

##### Remove the first or last string from variable
```bash
var="some string"
echo ${var:2}
#me string
```

##### Replacement (e.g. remove the first leading 0 )
```bash
var="0050"
echo ${var[@]#0}
#050
```

##### Replacement (e.g. replace 'a' with ',')
```bash
{var/a/,}
```

##### Replace all (e.g. replace all 'a' with ',')
```bash
{var//a/,}
```

##### Grep lines with strings from a file (e.g. lines with 'stringA or 'stringB' or 'stringC')
```bash
#with grep
test="stringA stringB stringC"
grep ${test// /\\\|} file.txt
# turning the space into 'or' (\|) in grep
```

##### To change the case of the string stored in the variable to lowercase (Parameter Expansion)
```bash
var=HelloWorld
echo ${var,,}
helloworld
```

##### Expand and then execute variable/argument
```bash
cmd="bar=foo"
eval "$cmd"
echo "$bar" # foo
```

## Math
[[back to top](#handy-bash-one-liners)]
##### Arithmetic Expansion in Bash (Operators: +, -, *, /, %, etc)
```bash
echo $(( 10 + 5 ))  #15
x=1
echo $(( x++ )) #1 , notice that it is still 1, since it's post-increment
echo $(( x++ )) #2
echo $(( ++x )) #4 , notice that it is not 3 since it's pre-increment
echo $(( x-- )) #4
echo $(( x-- )) #3
echo $(( --x )) #1
x=2
y=3
echo $(( x ** y )) #8
```

##### Print out the prime factors of a number (e.g. 50)
```bash
factor 50
# 50: 2 5 5
```
##### Sum up input list (e.g. seq 10)
```bash
seq 10|paste -sd+|bc
```

##### Sum up a file (each line in file contains only one number)
```bash
awk '{s+=$1} END {print s}' filename
```

##### Column subtraction
```bash
cat file| awk -F '\t' 'BEGIN {SUM=0}{SUM+=$3-$2}END{print SUM}'
```

##### Simple math with expr
```bash
expr 10+20 #30
expr 10\*20 #600
expr 30 \> 20 #1 (true)
```

##### More math with bc
```bash
# Number of decimal digit/ significant figure
echo "scale=2;2/3" | bc

# .66

# Exponent operator
echo "10^2" | bc
# 100

# Using variables
echo "var=5;--var"| bc
# 4
```


## Grep
[[back to top](#handy-bash-one-liners)]

#####  Type of grep
```bash
grep = grep -G # Basic Regular Expression (BRE)
fgrep = grep -F # fixed text, ignoring meta-characters
egrep = grep -E # Extended Regular Expression (ERE)
rgrep = grep -r # recursive
grep -P # Perl Compatible Regular Expressions (PCRE)
```

#####  Grep and count number of empty lines
```bash
grep -c "^$"
```

#####  Grep and return only integer
```bash
grep -o '[0-9]*'

#or
grep -oP '\d*'
```
#####  Grep integer with certain number of digits (e.g. 3)
```bash
grep '[0-9]\{3\}'

# or
grep -E '[0-9]{3}'

# or
grep -P '\d{3}'
```

#####  Grep only IP address
```bash
grep -Eo '[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}'

# or
grep -Po '\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}'
```

#####  Grep whole word (e.g. 'target')
```bash
grep -w 'target'

#or using RE
grep '\btarget\b'
```

#####  Grep returning lines before and after match (e.g. 'bbo')
```bash
# return also 3 lines after match
grep -A 3 'bbo'

# return also 3 lines before match
grep -B 3 'bbo'

# return also 3 lines before and after match
grep -C 3 'bbo'
```

#####  Grep string starting with (e.g. 'S')
```bash
grep -o 'S.*'
```

##### Extract text between words (e.g. w1,w2)
```bash
grep -o -P '(?<=w1).*(?=w2)'
```

##### Grep lines without word (e.g. 'bbo')
```bash
grep -v bbo filename
```

##### Grep lines not begin with string (e.g. #)
```bash
grep -v '^#' file.txt
```

##### Grep variables with space within it (e.g. myvar="some strings")
```bash
grep "$myvar" filename
#remember to quote the variable!
```

##### Grep only one/first match (e.g. 'bbo')
```bash
grep -m 1 bbo filename
```

##### Grep and return number of matching line(e.g. 'bbo')
```bash
grep -c bbo filename
```

##### Count occurrence (e.g. three times a line count three times)
```bash
grep -o bbo filename |wc -l
```

##### Case insensitive grep (e.g. 'bbo'/'BBO'/'Bbo')
```bash
grep -i "bbo" filename
```

##### COLOR the match (e.g. 'bbo')!
```bash
grep --color bbo filename
```

##### Grep search all files in a directory(e.g. 'bbo')
```bash
grep -R bbo /path/to/directory
# or
grep -r bbo /path/to/directory
```

##### Search all files in directory, do not ouput the filenames (e.g. 'bbo')
```bash
grep -rh bbo /path/to/directory
```

##### Search all files in directory, output ONLY the filenames with matches(e.g. 'bbo')
```bash
grep -rl bbo /path/to/directory
```

##### Grep OR (e.g. A or B or C or D)
```
grep 'A\|B\|C\|D'
```

##### Grep AND (e.g. A and B)
```bash
grep 'A.*B'
```

##### Regex any single character (e.g. ACB or AEB)
```bash
grep 'A.B'
```

##### Regex with or without a certain character (e.g. color or colour)
```bash
grep 'colou\?r'
```

##### Grep all content of a fileA from fileB
```bash
grep -f fileA fileB
```

##### Grep a tab
```bash
grep $'\t'
```

##### Grep variable from variable
```bash
$echo "$long_str"|grep -q "$short_str"
if [ $? -eq 0 ]; then echo 'found'; fi
#grep -q will output 0 if match found
#remember to add space between []!
```

##### Grep strings between a bracket()
```bash
grep -oP '\(\K[^\)]+'
```

##### Grep number of characters with known strings in between(e.g. AAEL000001-RA)
```bash
grep -o -w "\w\{10\}\-R\w\{1\}"
# \w word character [0-9a-zA-Z_] \W not word character
```

##### Skip directory (e.g. 'bbo')
```bash
grep -d skip 'bbo' /path/to/files/*
```



## Sed
[[back to top](#handy-bash-one-liners)]
##### Remove the 1st line
```bash
sed 1d filename
```

##### Remove the first 100 lines (remove line 1-100)
```bash
sed 1,100d filename
```

##### Remove lines with string (e.g. 'bbo')
```bash
sed "/bbo/d" filename
# case insensitive:
sed "/bbo/Id" filename
```

##### Remove lines whose nth character not equal to a value (e.g. 5th character not equal to 2)
```bash
sed -E '/^.{5}[^2]/d'
#aaaa2aaa (you can stay)
#aaaa1aaa (delete!)
```

##### Edit infile (edit and save to file), (e.g. deleting the lines with 'bbo' and save to file)
```bash
sed -i "/bbo/d" filename
```

##### When using variable (e.g. $i), use double quotes " "
```bash
# e.g. add >$i to the first line (to make a bioinformatics FASTA file)
sed "1i >$i"
# notice the double quotes! in other examples, you can use a single quote, but here, no way!
# '1i' means insert to first line
```

##### Using environment variable and end-of-line pattern at the same time.
```bash
# Use backslash for end-of-line $ pattern, and double quotes for expressing the variable
sed -e "\$s/\$/\n+--$3-----+/"
```

##### Delete/remove empty lines
```bash
sed '/^\s*$/d'

# or

sed '/^$/d'
```
##### Delete/remove last line
```bash
sed '$d'
```

##### Delete/remove last character from end of file
```bash
sed -i '$ s/.$//' filename
```

##### Add string to beginning of file (e.g. "\[")
```bash
sed -i '1s/^/[/' file
```

##### Add string at certain line number (e.g. add 'something' to line 1 and line 3)
```bash
sed -e '1isomething' -e '3isomething'
```

##### Add string to end of file (e.g. "]")
```bash
sed '$s/$/]/' filename
```
##### Add newline to the end
```bash
sed '$a\'
```

##### Add string to beginning of every line (e.g. 'bbo')
```bash
sed -e 's/^/bbo/' filename
```

##### Add string to end of each line (e.g. "}")
```bash
sed -e 's/$/\}\]/' filename
```

##### Add \n every nth character (e.g. every 4th character)
```bash
sed 's/.\{4\}/&\n/g'
```

##### Add a line after the line that matches the pattern (e.g. add a new line with "world" after the line with "hello")
```bash
sed '/hello*/a world' filename
# hello
# world
```

##### Concatenate/combine/join files with a separator and next line (e.g separate by ",")
```bash
sed -s '$a,' *.json > all.json
```

##### Substitution (e.g. replace A by B)
```bash
sed 's/A/B/g' filename
```

##### Substitution with wildcard (e.g. replace a line start with aaa= by aaa=/my/new/path)
```bash
sed "s/aaa=.*/aaa=\/my\/new\/path/g"
```

##### Select lines start with string (e.g. 'bbo')
```bash
sed -n '/^@S/p'
```
##### Delete lines with string (e.g. 'bbo')
```bash
sed '/bbo/d' filename
```

##### Print/get/trim a range of line (e.g. line 500-5000)
```bash
sed -n 500,5000p filename
```

##### Print every nth lines
```bash
sed -n '0~3p' filename

# catch 0: start; 3: step
```

##### Print every odd # lines
```bash
sed -n '1~2p'
```
##### Print every third line including the first line
```bash
sed -n '1p;0~3p'
```
##### Remove leading whitespace and tabs
```bash
sed -e 's/^[ \t]*//'
# Notice a whitespace before '\t'!!
```

##### Remove only leading whitespace
```bash
sed 's/ *//'

# notice a whitespace before '*'!!
```

##### Remove ending commas
```bash
sed 's/,$//g'
```

##### Add a column to the end
```bash
sed "s/$/\t$i/"
# $i is the valuable you want to add

# To add the filename to every last column of the file
for i in $(ls); do sed -i "s/$/\t$i/" $i; done
```

##### Add extension of filename to last column
```bash
for i in T000086_1.02.n T000086_1.02.p; do sed "s/$/\t${i/*./}/" $i; done >T000086_1.02.np
```

##### Remove newline\ nextline
```bash
sed ':a;N;$!ba;s/\n//g'
```

##### Print a particular line (e.g. 123th line)
```bash
sed -n -e '123p'
```

##### Print a number of lines (e.g. line 10th to line 33 rd)
```bash
sed -n '10,33p' <filename
```

##### Change delimiter
```bash
sed 's=/=\\/=g'
```

##### Replace with wildcard (e.g A-1-e or A-2-e or A-3-e....)
```bash
sed 's/A-.*-e//g' filename
```

##### Remove last character of file
```bash
sed '$ s/.$//'
```

##### Insert character at specified position of file (e.g. AAAAAA --> AAA#AAA)
```bash
sed -r -e 's/^.{3}/&#/' file
```


## Awk
[[back to top](#handy-bash-one-liners)]

##### Set tab as field separator
```bash
awk -F $'\t'
```

##### Output as tab separated (also as field separator)
```bash
awk -v OFS='\t'
```

##### Pass variable
```bash
a=bbo;b=obb;
awk -v a="$a" -v b="$b" "$1==a && $10=b" filename
```

##### Print line number and number of characters on each line
```bash
awk '{print NR,length($0);}' filename
```

##### Find number of columns
```bash
awk '{print NF}'
```

##### Reverse column order
```bash
awk '{print $2, $1}'
```

##### Check if there is a comma in a column (e.g. column $1)
```bash
awk '$1~/,/ {print}'
```

##### Split and do for loop
```bash
awk '{split($2, a,",");for (i in a) print $1"\t"a[i]}' filename
```

##### Print all lines before nth occurrence of a string (e.g stop print lines when 'bbo' appears 7 times)
```bash
awk -v N=7 '{print}/bbo/&& --N<=0 {exit}'
```

##### Print filename and last line of all files in directory
```bash
ls|xargs -n1 -I file awk '{s=$0};END{print FILENAME,s}' file
```

##### Add string to the beginning of a column (e.g add "chr" to column $3)
```bash
awk 'BEGIN{OFS="\t"}$3="chr"$3'
```

##### Remove lines with string (e.g. 'bbo')
```bash
awk '!/bbo/' file
```

##### Remove last column
```bash
awk 'NF{NF-=1};1' file
```

##### Usage and meaning of NR and FNR
```bash
# For example there are two files:
# fileA:
# a
# b
# c
# fileB:
# d
# e
awk 'print FILENAME, NR,FNR,$0}' fileA fileB
# fileA    1    1    a
# fileA    2    2    b
# fileA    3    3    c
# fileB    4    1    d
# fileB    5    2    e
```

##### AND gate
```bash
# For example there are two files:
# fileA:
# 1    0
# 2    1
# 3    1
# 4    0
# fileB:
# 1    0
# 2    1
# 3    0
# 4    1

awk -v OFS='\t' 'NR=FNR{a[$1]=$2;next} NF {print $1,((a[$1]=$2)? $2:"0")}' fileA fileB
# 1    0
# 2    1
# 3    0
# 4    0
```

##### Round all numbers of file (e.g. 2 significant figure)
```bash
awk '{while (match($0, /[0-9]+\[0-9]+/)){
    \printf "%s%.2f", substr($0,0,RSTART-1),substr($0,RSTART,RLENGTH)
    \$0=substr($0, RSTART+RLENGTH)
    \}
    \print
    \}'
```

##### Give number/index to every row
```bash
awk '{printf("%s\t%s\n",NR,$0)}'
```

##### Break combine column data into rows
```bash
# For example, separate the following content:
# David    cat,dog
# into
# David    cat
# David    dog

awk '{split($2,a,",");for(i in a)print $1"\t"a[i]}' file

# Detail here:　http://stackoverflow.com/questions/33408762/bash-turning-single-comma-separated-column-into-multi-line-string
```

##### Average a file (each line in file contains only one number)
```bash
awk '{s+=$1}END{print s/NR}'
```

##### Print field start with string (e.g Linux)
```bash
awk '$1 ~ /^Linux/'
```

##### Sort a row (e.g. 1 40  35  12  23  --> 1 12    23  35  40)
```bash
awk ' {split( $0, a, "\t" ); asort( a ); for( i = 1; i <= length(a); i++ ) printf( "%s\t", a[i] ); printf( "\n" ); }'
```

##### Subtract previous row values (add column6 which equal to column4 minus last column5)
```bash
awk '{$6 = $4 - prev5; prev5 = $5; print;}'
```

## Xargs
[[back to top](#handy-bash-one-liners)]

##### Set tab as delimiter (default:space)
```bash
xargs -d\t
```

##### Prompt commands before running commands
```bash
ls|xargs -L1 -p head
```

##### Display 3 items per line
```bash
echo 1 2 3 4 5 6| xargs -n 3
# 1 2 3
# 4 5 6

```
##### Prompt before execution
```bash
echo a b c |xargs -p -n 3
```

##### Print command along with output
```bash
xargs -t abcd
# bin/echo abcd
# abcd

```
##### With find and rm
```bash
find . -name "*.html"|xargs rm

# when using a backtick
rm `find . -name "*.html"`
```

##### Delete files with whitespace in filename (e.g. "hello 2001")
```bash
find . -name "*.c" -print0|xargs -0 rm -rf
```

##### Show limits on command-line length
```bash
xargs --show-limits
# Output from my Ubuntu:
# Your environment variables take up 3653 bytes
# POSIX upper limit on argument length (this system): 2091451
# POSIX smallest allowable upper limit on argument length (all systems): 4096
# Maximum length of command we could actually use: 2087798
# Size of command buffer we are actually using: 131072
# Maximum parallelism (--max-procs must be no greater): 2147483647
```

##### Move files to folder
```bash
find . -name "*.bak" -print 0|xargs -0 -I {} mv {} ~/old

# or
find . -name "*.bak" -print 0|xargs -0 -I file mv file ~/old
```

##### Move first 100th files to a directory (e.g. d1)
```bash
ls |head -100|xargs -I {} mv {} d1
```

##### Parallel
```bash
time echo {1..5} |xargs -n 1 -P 5 sleep

# a lot faster than:
time echo {1..5} |xargs -n1 sleep
```

##### Copy all files from A to B
```bash
find /dir/to/A -type f -name "*.py" -print 0| xargs -0 -r -I file cp -v -p file --target-directory=/path/to/B

# v: verbose|
# p: keep detail (e.g. owner)

```

##### With sed
```bash
ls |xargs -n1 -I file sed -i '/^Pos/d' file
```

##### Add the file name to the first line of file
```bash
ls |sed 's/.txt//g'|xargs -n1 -I file sed -i -e '1 i\>file\' file.txt
```

##### Count all files
```bash
ls |xargs -n1 wc -l
```

##### Turn output into a single line
```bash
ls -l| xargs
```

##### Count files within directories
```bash
echo mso{1..8}|xargs -n1 bash -c 'echo -n "$1:"; ls -la "$1"| grep -w 74 |wc -l' --
# "--" signals the end of options and display further option processing
```

##### Count lines in all file, also count total lines
```bash
ls|xargs wc -l
```
##### Xargs and grep
```bash
cat grep_list |xargs -I{} grep {} filename
```

##### Xargs and sed (replace all old ip address with new ip address under /etc directory)
```bash
grep -rl '192.0.2.1' /etc | xargs sed -i 's/192.0.2.1/192.0.2.2/g'
```


## Find
[[back to top](#handy-bash-one-liners)]
##### List all sub directory/file in the current directory
```bash
find .
```

##### List all files under the current directory
```bash
find . -type f
```

##### List all directories under the current directory
```bash
find . -type d
```

##### Edit all files under current directory (e.g. replace 'www' with 'ww')
```bash
find . -name '*.php' -exec sed -i 's/www/w/g' {} \;

# if there are no subdirectory
replace "www" "w" -- *
# a space before *
```
##### Find and output only filename (e.g. "mso")
```bash
find mso*/ -name M* -printf "%f\n"
```

##### Find large files in the system (e.g. >4G)
```bash
find / -type f -size +4G
```

##### Find and delete file with size less than (e.g. 74 byte)
```bash
find . -name "*.mso" -size -74c -delete

# M for MB, etc
```

##### Find empty (0 byte) files
```bash
find . -type f -empty
# to further delete all the empty files
find . -type f -empty -delete
```

##### Recursively count all the files in a directory
```bash
find . -type f | wc -l
```

## Condition and loop
[[back to top](#handy-bash-one-liners)]

##### If statement
```bash
# if and else loop for string matching
if [[ "$c" == "read" ]]; then outputdir="seq"; else outputdir="write" ; fi

# Test if myfile contains the string 'test':
if grep -q hello myfile; then echo -e "file contains the string!" ; fi

# Test if mydir is a directory, change to it and do other stuff:
if cd mydir; then
  echo 'some content' >myfile
else
  echo >&2 "Fatal error. This script requires mydir."
fi

# if variable is null
if [ ! -s "myvariable" ]; then echo -e "variable is null!" ; fi
#True of the length if "STRING" is zero.

# Using test command (same as []), to test if the length of variable is nonzero
test -n "$myvariable" && echo myvariable is "$myvariable" || echo myvariable is not set

# Test if file exist
if [ -e 'filename' ]
then
  echo -e "file exists!"
fi

# Test if file exist but also including symbolic links:
if [ -e myfile ] || [ -L myfile ]
then
  echo -e "file exists!"
fi

# Test if the value of x is greater or equal than 5
if [ "$x" -ge 5 ]; then echo -e "greater or equal than 5!" ; fi

# Test if the value of x is greater or equal than 5, in bash/ksh/zsh:
if ((x >= 5)); then echo -e "greater or equal than 5!" ; fi

# Use (( )) for arithmetic operation
if ((j==u+2)); then echo -e "j==u+2!!" ; fi

# Use [[ ]] for comparison
if [[ $age -gt 21 ]]; then echo -e "forever 21!!" ; fi

```

[More if commands](http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_07_01.html)

##### For loop
```bash
# Echo the file name under the current directory
for i in $(ls); do echo file $i; done
#or
for i in *; do echo file $i; done

# Make directories listed in a file (e.g. myfile)
for dir in $(<myfile); do mkdir $dir; done

# Press any key to continue each loop
for i in $(cat tpc_stats_0925.log |grep failed|grep -o '\query\w\{1,2\}'); do cat ${i}.log; read -rsp $'Press any key to continue...\n' -n1 key; done

# Print a file line by line when a key is pressed,
oifs="$IFS"; IFS=$'\n'; for line in $(cat myfile); do ...; done
while read -r line; do ...; done <myfile

#If only one word a line, simply
for line in $(cat myfile); do echo $line; read -n1; done

#Loop through an array
for i in "${arrayName[@]}"; do echo $i; done

```

##### While loop,
```bash
# Column subtraction of a file (e.g. a 3 columns file)
while read a b c; do echo $(($c-$b)); done < <(head filename)
#there is a space between the two '<'s

# Sum up column subtraction
i=0; while read a b c; do ((i+=$c-$b)); echo $i; done < <(head filename)

# Keep checking a running process (e.g. perl) and start another new process (e.g. python) immediately after it. (BETTER use the wait command! Ctrl+F 'wait')
while [[ $(pidof perl) ]]; do echo f; sleep 10; done && python timetorunpython.py
```

##### switch (case in bash)
```bash
read type;
case $type in
  '0')
    echo 'how'
    ;;
  '1')
    echo 'are'
    ;;
  '2')
    echo 'you'
    ;;
esac
```

## Time
[[back to top](#handy-bash-one-liners)]

##### Find out the time require for executing a command
```bash
time echo hi
```

##### Wait for some time (e.g 10s)
```bash
sleep 10
```

##### Print date with formatting
```bash
date +%F
# 2020-07-19

# or
date +'%d-%b-%Y-%H:%M:%S'
# 10-Apr-2020-21:54:40

# Returns the current time with nanoseconds.
date +"%T.%N"
# 11:42:18.664217000  

# Get the seconds since epoch (Jan 1 1970) for a given date (e.g Mar 16 2021)
date -d "Mar 16 2021" +%s
# 1615852800
# or
date -d "Tue Mar 16 00:00:00 UTC 2021"  +%s
# 1615852800  

# Convert the number of seconds since epoch back to date
date --date @1615852800
# Tue Mar 16 00:00:00 UTC 2021

```

##### Print current time point for N days ago or N days after
```bash
# print current date first (for the following example)
date +"%F %H:%M:%S"
# 2023-03-11 16:17:09

# print the time that is 1 day ago
date -d"1 day ago" +"%F %H:%M:%S"
# 2023-03-10 16:17:09

# print the time that is 7 days ago
date -d"7 days ago" +"%F %H:%M:%S"
# 2023-03-04 16:17:09

# print the time that is a week ago
date -d"1 week ago" +"%F %H:%M:%S"
# 2023-03-04 16:17:09

# add 1 day to date
date -d"-1 day ago" +"%F %H:%M:%S"
# 2023-03-12 16:17:09
```

##### wait for random duration (e.g. sleep 1-5 second, like adding a jitter)
```bash
sleep $[ ( $RANDOM % 5 ) + 1 ]
```

##### Log out your account after a certain period of time (e.g 10 seconds)
```bash
TMOUT=10
#once you set this variable, logout timer start running!
```

##### Set how long you want to run a command
```bash
#This will run the command 'sleep 10' for only 1 second.
timeout 1 sleep 10
```

##### Set when you want to run a command (e.g 1 min from now)
```bash
at now + 1min  #time-units can be minutes, hours, days, or weeks
warning: commands will be executed using /bin/sh
at> echo hihigithub >~/itworks
at> <EOT>   # press Ctrl + D to exit
job 1 at Wed Apr 18 11:16:00 2018
```


## Download
[[back to top](#handy-bash-one-liners)]

##### Download the content of this README.md (the one your are viewing now)
```bash
curl https://raw.githubusercontent.com/onceupon/Bash-Oneliner/master/README.md | pandoc -f markdown -t man | man -l -

# or w3m (a text based web browser and pager)
curl https://raw.githubusercontent.com/onceupon/Bash-Oneliner/master/README.md | pandoc | w3m -T text/html

# or using emacs (in emac text editor)
emacs --eval '(org-mode)' --insert <(curl https://raw.githubusercontent.com/onceupon/Bash-Oneliner/master/README.md | pandoc -t org)

# or using emacs (on terminal, exit using Ctrl + x then Ctrl + c)
emacs -nw --eval '(org-mode)' --insert <(curl https://raw.githubusercontent.com/onceupon/Bash-Oneliner/master/README.md | pandoc -t org)
```

##### Download all from a page
```bash
wget -r -l1 -H -t1 -nd -N -np -A mp3 -e robots=off http://example.net

# -r: recursive and download all links on page
# -l1: only one level link
# -H: span host, visit other hosts
# -t1: numbers of retries
# -nd: don't make new directories, download to here
# -N: turn on timestamp
# -nd: no parent
# -A: type (separate by ,)
# -e robots=off: ignore the robots.txt file which stop wget from crashing the site, sorry example.net
```

##### Upload a file to web and download (https://transfer.sh/)
```bash
#  Upload a file (e.g. filename.txt):
curl --upload-file ./filename.txt https://transfer.sh/filename.txt
# the above command will return a URL, e.g: https://transfer.sh/tG8rM/filename.txt

# Next you can download it by:
curl https://transfer.sh/tG8rM/filename.txt -o filename.txt
```

##### Download file if necessary
```bash
data=file.txt
url=http://www.example.net/$data
if [ ! -s $data ];then
    echo "downloading test data..."
    wget $url
fi
```

##### Wget to a filename (when a long name)
```bash
wget -O filename "http://example.net"
```

##### Wget files to a folder
```bash
wget -P /path/to/directory "http://example.net"
```

##### Instruct curl to follow any redirect until it reaches the final destination:
```bash
curl -L example.net
```

## Random
[[back to top](#handy-bash-one-liners)]
##### Random generate password (e.g. generate 5 password each of length 13)
```bash
sudo apt install pwgen
pwgen 13 5
#sahcahS9dah4a xieXaiJaey7xa UuMeo0ma7eic9 Ahpah9see3zai acerae7Huigh7
```

##### Random pick 100 lines from a file
```bash
shuf -n 100 filename
```

##### Random order (lucky draw)
```bash
for i in a b c d e; do echo $i; done | shuf
```

##### Echo series of random numbers between a range (e.g. shuffle numbers from 0-100, then pick 15 of them randomly)
```bash
shuf -i 0-100 -n 15
```

##### Echo a random number
```bash
echo $RANDOM
```

##### Random from 0-9
```bash
echo $((RANDOM % 10))
```

##### Random from 1-10
```bash
echo $(((RANDOM %10)+1))
```

## Xwindow
[[back to top](#handy-bash-one-liners)]

X11 GUI applications! Here are some GUI tools for you if you get bored by the text-only environment.

##### Enable X11 forwarding,in order to use graphical application on servers
```bash
ssh -X user_name@ip_address

# or setting through xhost
# --> Install the following for Centos:
# xorg-x11-xauth
# xorg-x11-fonts-*
# xorg-x11-utils
```

##### Little xwindow tools
```bash
xclock
xeyes
xcowsay
```

##### Open pictures/images from ssh server
```bash
1. ssh -X user_name@ip_address
2. apt-get install eog
3. eog picture.png
```

##### Watch videos on server
```bash
1. ssh -X user_name@ip_address
2. sudo apt install mpv
3. mpv myvideo.mp4
```

##### Use gedit on server (GUI editor)
```bash
1. ssh -X user_name@ip_address
2. apt-get install gedit
3. gedit filename.txt
```

##### Open PDF file from ssh server
```bash
1. ssh -X user_name@ip_address
2. apt-get install evince
3. evince filename.pdf
```

##### Use google-chrome browser from ssh server (Spyware, so don't)
```bash
1. ssh -X user_name@ip_address
2. apt-get install libxss1 libappindicator1 libindicator7
3. wget https://dl.example.net/linux/direct/google-chrome-stable_current_amd64.deb
4. sudo apt-get install -f
5. dpkg -i google-chrome*.deb
6. google-chrome
 ```


## System
[[back to top](#handy-bash-one-liners)]

##### Work with yum history
```bash
# List yum history (e.g install, update)
sudo yum history
# Example output:
# Loaded plugins: extras_suggestions, langpacks, priorities, update-motd
# ID     | Login user               | Date and time    | Action(s)      | Altered
# -------------------------------------------------------------------------------
#     11 |  ... <myuser>       | 2020-04-10 10:57 | Install        |    1 P<
#     10 |  ... <myuser>       | 2020-03-27 05:21 | Install        |    1 >P
#      9 |  ... <myuser>       | 2020-03-05 11:57 | I, U           |   56 *<
# ...

# Show more details of a yum history (e.g. history #11)
sudo yum history info 11

# Undo a yum history (e.g. history #11, this will uninstall some packages)
sudo yum history undo 11
```

##### Audit files to see who made changes to a file [RedHat based system only]
```bash
# To audit a directory recursively for changes (e.g. myproject)
auditctl -w /path/to/myproject/ -p wa

# If you delete a file name "VIPfile", the deletion is recorded in /var/log/audit/audit.log
sudo grep VIPfile /var/log/audit/audit.log
#type=PATH msg=audit(1581417313.678:113): item=1 name="VIPfile" inode=300115 dev=ca:01 mode=0100664 ouid=1000 ogid=1000 rdev=00:00 nametype=DELETE cap_fp=0000000000000000 cap_fi=0000000000000000 cap_fe=0 cap_fver=0
```

##### Check out whether SELinux is enabled
```bash
sestatus
# SELinux status:                 enabled
# SELinuxfs mount:                /sys/fs/selinux
# SELinux root directory:         /etc/selinux
# Loaded policy name:             targeted
# Current mode:                   enforcing
# Mode from config file:          enforcing
# Policy MLS status:              enabled
# Policy deny_unknown status:     allowed
# Max kernel policy version:      31
```

##### Generate public key from private key
```bash
ssh-keygen -y -f ~/.ssh/id_rsa > ~/.ssh/id_rsa.pub
```

##### Copy your default public key to remote user
```bash
ssh-copy-id <user_name>@<server_IP>
# then you need to enter the password
# and next time you won't need to enter password when ssh to that user
```

##### Copy default public key to remote user using the required private key (e.g. use your mykey.pem key to copy your id_rsa.pub to the remote user)
```bash
# before you need to use mykey.pem to ssh to remote user.
ssh-copy-id -i ~/.ssh/id_rsa.pub -o "IdentityFile ~/Downloads/mykey.pem" <user_name>@<server_IP>
# now you don't need to use key to ssh to that user.
```

##### SSH Agent Forwarding
```bash
# To bring your key with you when ssh to serverA, then ssh to serverB from serverA using the key.
ssh-agent
ssh-add /path/to/mykey.pem
ssh -A <username>@<IP_of_serverA>
# Next you can ssh to serverB
ssh <username>@<IP_of_serverB>
```

##### Set the default user and key for a host when using SSH
```bash
# add the following to ~/.ssh/config
Host myserver
  User myuser
  IdentityFile ~/path/to/mykey.pem

# Next, you could run "ssh myserver" instead of "ssh -i ~/path/to/mykey.pem myuser@myserver"
```

##### Follow the most recent logs from service
```bash
journalctl -u <service_name> -f
```

##### Eliminate the zombie
```bash
# A zombie is already dead, so you cannot kill it. You can eliminate the zombie by killing its parent.
# First, find PID of the zombie
ps aux| grep 'Z'
# Next find the PID of zombie's parent
pstree -p -s <zombie_PID>
# Then you can kill its parent and you will notice the zombie is gone.
sudo kill 9 <parent_PID>
```
###### Show memory usage
```bash
free -c 10 -mhs 1
# print 10 times, at 1 second interval
```

##### Display CPU and IO statistics for devices and partitions.
```bash
# refresh every second
iostat -x -t 1
```

##### Display bandwidth usage on an network interface (e.g. enp175s0f0)
```bash
iftop -i enp175s0f0
```

##### Tell how long the system has been running and number of users
```bash
uptime
```

##### Check if it's root running
```bash
if [ "$EUID" -ne 0 ]; then
        echo "Please run this as root"
        exit 1
fi
```
##### Change shell of a user (e.g. bonnie)
```bash
chsh -s /bin/sh bonnie
# /etc/shells: valid login shells
```

##### Change root / fake root / jail (e.g. change root to newroot)
```bash
chroot /home/newroot /bin/bash

# To exit chroot
exit
```
##### Display file status (size; access, modify and change time, etc) of a file (e.g. filename.txt)
```bash
stat filename.txt
```

##### Snapshot of the current processes
```bash
ps aux
```

##### Display a tree of processes
```bash
pstree
```

##### Find maximum number of processes
```bash
cat /proc/sys/kernel/pid_max
```

##### Print or control the kernel ring buffer
```bash
dmesg
```

##### Show IP address
```bash
$ip add show

# or
ifconfig
```

##### Print previous and current SysV runlevel
```bash
runlevel

# or
who -r
```

##### Change SysV runlevel (e.g. 5)
```bash
init 5
#or
telinit 5
```

##### Display all available services in all runlevels,
```bash
chkconfig --list
# update-rc.d equivalent to chkconfig in ubuntu
```

##### Check system version
```bash
cat /etc/*-release
```

##### Linux Programmer's Manuel: hier- description of the filesystem hierarchy
```bash
man hier
```

##### Control the systemd system and service manager
```bash
# e.g. check the status of cron service
systemctl status cron.service

# e.g. stop cron service
systemctl stop cron.service
```

##### List job
```bash
jobs -l
```

##### Run a program with modified priority (e.g. ./test.sh)
```bash
# nice value is adjustable from -20 (most favorable) to +19
# the nicer the application, the lower the priority
# Default niceness: 10; default priority: 80

nice -10 ./test.sh
```

##### Export PATH
```bash
export PATH=$PATH:~/path/you/want
```

##### Make file executable
```bash
chmod +x filename
# you can now ./filename to execute it
```

##### Print system information
```bash
uname -a

# Check system hardware-platform (x86-64)
uname -i
```

##### Surf the net
```bash
links www.example.net
```

##### Add user, set passwd
```bash
useradd username
passwd username
```

##### Edit PS1 variable for bash (e.g. displaying the whole path)
```bash
1. vi ~/.bash_profile
2. export PS1='\u@\h:\w\$'
# $PS1 is a variable that defines the makeup and style of the command prompt
# You could use emojis and add timestamp to every prompt using the following value:
# export PS1="\t@🦁:\w\$ "
3. source ~/.bash_profile
```

##### Edit environment setting (e.g. alias)
```bash
1. vi ~/.bash_profile
2. alias pd="pwd" //no more need to type that 'w'!
3. source ~/.bash_profile
```
##### Print all alias
```bash
alias -p
```

##### Unalias (e.g. after alias ls='ls --color=auto')
```bash
unalias ls
```

##### Set and unset shell options
```bash
# print all shell options
shopt

# to unset (or stop) alias
shopt -u expand_aliases

# to set (or start) alias
shopt -s expand_aliases
```

##### List environment variables (e.g. PATH)
```bash
echo $PATH
# list of directories separated by a colon
```
##### List all environment variables for current user
```bash
env
```
##### Unset environment variable (e.g. unset variable 'MYVAR')
```bash
unset MYVAR
```

##### Show partition format
```bash
lsblk
```

##### Inform the OS of partition table changes
```bash
partprobe
```

##### Soft link program to bin
```bash
ln -s /path/to/program /home/usr/bin
# must be the whole path to the program
```

##### Show hexadecimal view of data
```bash
hexdump -C filename.class
```

##### Jump to different node
```bash
rsh node_name
```

##### Check port (active internet connection)
```bash
netstat -tulpn
```

##### Print resolved symbolic links or canonical file names
```bash
readlink filename
```

##### Find out the type of command and where it link to (e.g. python)
```bash
type python
# python is /usr/bin/python
# There are 5 different types, check using the 'type -f' flag
# 1. alias    (shell alias)
# 2. function (shell function, type will also print the function body)
# 3. builtin  (shell builtin)
# 4. file     (disk file)
# 5. keyword  (shell reserved word)

# You can also use `which`
which python
# /usr/bin/python
```

##### List all functions names
```bash
declare -F
```

##### List total size of a directory
```bash
du -hs .

# or
du -sb
```

##### Copy directory with permission setting
```bash
cp -rp /path/to/directory
```

##### Store current directory
```bash
pushd .

# then pop
popd

#or use dirs to display the list of currently remembered directories.
dirs -l
```

##### Show disk usage
```bash
df -h

# or
du -h

#or
du -sk /var/log/* |sort -rn |head -10
```

##### check the Inode utilization
```
df -i
# Filesystem      Inodes  IUsed   IFree IUse% Mounted on
# devtmpfs        492652    304  492348    1% /dev
# tmpfs           497233      2  497231    1% /dev/shm
# tmpfs           497233    439  496794    1% /run
# tmpfs           497233     16  497217    1% /sys/fs/cgroup
# /dev/nvme0n1p1 5037976 370882 4667094    8% /
# tmpfs           497233      1  497232    1% /run/user/1000
```

##### Show all file system type
```bash
df -TH
```

##### Show current runlevel
```bash
runlevel
```

##### Switch runlevel
```bash
init 3

#or
telinit 3
```

##### Permanently modify runlevel
```bash
1. edit /etc/init/rc-sysinit.conf
2. env DEFAULT_RUNLEVEL=2
```

##### Become root
```bash
su
```

##### Become somebody
```bash
su somebody
```

##### Report user quotes on device
```bash
repquota -auvs
```

##### Get entries in a number of important databases
```bash
getent database_name

# (e.g. the 'passwd' database)
getent passwd
# list all user account (all local and LDAP)

# (e.g. fetch list of grop accounts)
getent group
# store in database 'group'
```

##### Change owner of file
```bash
chown user_name filename
chown -R user_name /path/to/directory/
# chown user:group filename
```

##### Mount and unmount
```bash
# e.g. Mount /dev/sdb to /home/test
mount /dev/sdb /home/test

# e.g. Unmount /home/test
umount /home/test
```

##### List current mount detail
```bash
mount
# or
df
```

##### List current usernames and user-numbers
```bash
cat /etc/passwd
```

##### Get all username
```bash
getent passwd| awk '{FS="[:]"; print $1}'
```

##### Show all users
```bash
compgen -u
```

##### Show all groups
```bash
compgen -g
```

##### Show group of user
```bash
group username
```

##### Show uid, gid, group of user
```bash
id username

# variable for UID
echo $UID
```

##### Check if it's root
```bash
if [ $(id -u) -ne 0 ];then
    echo "You are not root!"
    exit;
fi
# 'id -u' output 0 if it's not root
```

##### Find out CPU information
```bash
more /proc/cpuinfo

# or
lscpu
```

##### Set quota for user (e.g. disk soft limit: 120586240; hard limit: 125829120)
```bash
setquota username 120586240 125829120 0 0 /home
```

##### Show quota for user
```bash
quota -v username
```

##### Display current libraries from the cache
```bash
ldconfig -p
```

##### Print shared library dependencies (e.g. for 'ls')
```bash
ldd /bin/ls
```

##### Check user login
```bash
lastlog
```
##### Check last reboot history
```bash
last reboot
```

##### Edit path for all users
```bash
joe /etc/environment
# edit this file
```

##### Show and set user limit
```bash
ulimit -u
```

##### Print out number of cores/ processors
```bash
nproc --all
```

##### Check status of each core
```
1. top
2. press '1'
```

##### Show jobs and PID
```bash
jobs -l
```

##### List all running services
```bash
service --status-all
```

##### Schedule shutdown server
```bash
shutdown -r +5 "Server will restart in 5 minutes. Please save your work."
```

##### Cancel scheduled shutdown
```bash
shutdown -c
```

##### Broadcast to all users
```bash
wall -n hihi
```

##### Kill all process of a user
```bash
pkill -U user_name
```

##### Kill all process of a program
```bash
kill -9 $(ps aux | grep 'program_name' | awk '{print $2}')
```

##### Set gedit preference on server
```
# You might have to install the following:

apt-get install libglib2.0-bin;
# or
yum install dconf dconf-editor;
yum install dbus dbus-x11;

# Check list
gsettings list-recursively

# Change some settings
gsettings set org.gnome.gedit.preferences.editor highlight-current-line true
gsettings set org.gnome.gedit.preferences.editor scheme 'cobalt'
gsettings set org.gnome.gedit.preferences.editor use-default-font false
gsettings set org.gnome.gedit.preferences.editor editor-font 'Cantarell Regular 12'
```
##### Add user to a group (e.g add user 'nice' to the group 'docker', so that he can run docker without sudo)
```bash
sudo gpasswd -a nice docker
```

##### Pip install python package without root
```bash
1. pip install --user package_name
2. You might need to export ~/.local/bin/ to PATH: export PATH=$PATH:~/.local/bin/
```

##### Removing old linux kernels (when /boot almost full...)
```bash
1. uname -a  #check current kernel, which should NOT be removed
2. sudo apt-get purge linux-image-X.X.X-X-generic  #replace old version
```


##### Change hostname
```bash
sudo hostname your-new-name

# if not working, do also:
hostnamectl set-hostname your-new-hostname
# then check with:
hostnamectl
# Or check /etc/hostname

# If still not working..., edit:
/etc/sysconfig/network
/etc/sysconfig/network-scripts/ifcfg-ensxxx
#add HOSTNAME="your-new-hostname"
 ```

##### List installed packages
```bash
apt list --installed

# or on Red Hat:
yum list installed
```

##### Check for package update
```bash
apt list --upgradeable

# or
sudo yum check-update
```

##### Run yum update excluding a package (e.g. do not update php packages)
```bash
sudo yum update --exclude=php*
```

##### Check which file make the device busy on umount
```bash
lsof /mnt/dir
```

##### When sound not working
```bash
killall pulseaudio
# then press Alt-F2 and type in pulseaudio
```

##### List information about SCSI devices
```bash
lsscsi
```

##### Tutorial for setting up your own DNS server
http://onceuponmine.blogspot.tw/2017/08/set-up-your-own-dns-server.html

##### Tutorial for creating a simple daemon
http://onceuponmine.blogspot.tw/2017/07/create-your-first-simple-daemon.html

##### Tutorial for using your gmail to send email
http://onceuponmine.blogspot.tw/2017/10/setting-up-msmtprc-and-use-your-gmail.html

##### Using telnet to test open ports, test if you can connect to a port (e.g 53) of a server (e.g 192.168.2.106)
```bash
telnet 192.168.2.106 53
```

##### Change network maximum transmission unit (mtu) (e.g. change to 9000)
```bash
ifconfig eth0 mtu 9000
```

##### Get pid of a running process (e.g python)
```bash
pidof python

# or
ps aux|grep python
```

##### Check status of a process using PID
```bash
ps -p <PID>

#or
cat /proc/<PID>/status
cat /proc/<PID>/stack
cat /proc/<PID>/stat
```

##### NTP
```bash
# Start ntp:
ntpd

# Check ntp:
ntpq -p
```

##### Remove unnecessary files to clean your server
```bash
sudo apt-get autoremove
sudo apt-get clean
sudo rm -rf ~/.cache/thumbnails/*

# Remove old kernal:
sudo dpkg --list 'linux-image*'
sudo apt-get remove linux-image-OLDER_VERSION
```

##### Increase/ resize root partition (root partition is an LVM logical volume)
```bash
pvscan
lvextend -L +130G /dev/rhel/root -r
# Adding -r will grow filesystem after resizing the volume.
```

##### Create a UEFI Bootable USB drive (e.g. /dev/sdc1)
```bash
sudo dd if=~/path/to/isofile.iso of=/dev/sdc1 oflag=direct bs=1048576
```

##### Locate and remove a package
```bash
sudo dpkg -l | grep <package_name>
sudo dpkg --purge <package_name>
```

##### Create a ssh tunnel
```bash
ssh -f -L 9000:targetservername:8088 root@192.168.14.72 -N
#-f: run in background; -L: Listen; -N: do nothing
#the 9000 of your computer is now connected to the 8088 port of the targetservername through 192.168.14.72
#so that you can see the content of targetservername:8088 by entering localhost:9000 from your browser.
```
##### Get process ID of a process (e.g. sublime_text)
```bash
#pidof
pidof sublime_text

#pgrep, you don't have to type the whole program name
pgrep sublim

#pgrep, echo 1 if process found, echo 0 if no such process
pgrep -q sublime_text && echo 1 || echo 0

#top, takes longer time
top|grep sublime_text
```

##### Some benchmarking tools for your server
[aio-stress](https://openbenchmarking.org/test/pts/aio-stress) - AIO benchmark.  
[bandwidth](https://zsmith.co/bandwidth.html) - memory bandwidth benchmark.  
[bonnie++](https://www.coker.com.au/bonnie++/) - hard drive and file system performance benchmark.  
[dbench](https://dbench.samba.org/) -  generate I/O workloads to either a filesystem or to a networked CIFS or NFS server.  
[dnsperf](https://www.dnsperf.com/) - authorative and recursing DNS servers.  
[filebench](https://github.com/filebench/filebench) - model based file system workload generator.  
[fio](https://linux.die.net/man/1/fio) - I/O  benchmark.  
[fs_mark](https://github.com/josefbacik/fs_mark) - synchronous/async file creation benchmark.  
[httperf](https://github.com/httperf/httperf) - measure web server performance.  
[interbench](https://github.com/ckolivas/interbench) - linux interactivity  benchmark.  
[ioblazer](https://labs.vmware.com/flings/ioblazer) - multi-platform storage stack micro-benchmark.  
[iozone](http://www.iozone.org/) - filesystem benchmark.  
[iperf3](https://iperf.fr/iperf-download.php) - measure TCP/UDP/SCTP performance.  
[kcbench](https://github.com/knurd/kcbench) - kernel compile benchmark, compiles a kernel and measures the time it takes.  
[lmbench](http://www.bitmover.com/lmbench/) - Suite of simple, portable benchmarks.  
[netperf](https://github.com/HewlettPackard/netperf) - measure network performance, test unidirectional throughput, and end-to-end latency.  
[netpipe](https://linux.die.net/man/1/netpipe) - network protocol independent performance evaluator.  
[nfsometer](http://wiki.linux-nfs.org/wiki/index.php/NFSometer) - NFS performance framework.  
[nuttcp](https://www.nuttcp.net/Welcome%20Page.html) - measure network performance.  
[phoronix-test-suite](https://www.phoronix-test-suite.com/) - comprehensive automated testing and benchmarking platform.  
[seeker](https://github.com/fidlej/seeker) - portable disk seek benchmark.  
[siege](https://github.com/JoeDog/siege) - http load tester and benchmark.  
[sockperf](https://github.com/Mellanox/sockperf) - network benchmarking utility over socket API.  
[spew](https://linux.die.net/man/1/spew) - measures I/O performance and/or generates I/O load.  
[stress](https://people.seas.harvard.edu/~apw/stress/) - workload generator for POSIX systems.  
[sysbench](https://github.com/akopytov/sysbench) - scriptable database and system performance benchmark.  
[tiobench](https://github.com/mkuoppal/tiobench) - threaded IO benchmark.  
[unixbench](https://github.com/kdlucas/byte-unixbench) - the original BYTE UNIX benchmark suite, provide a basic indicator of the performance of a Unix-like system.  
[wrk](https://github.com/wg/wrk) - HTTP benchmark.  


##### Performance monitoring tool - sar
```bash
# installation
# It collects the data every 10 minutes and generate its report daily. crontab file (/etc/cron.d/sysstat) is responsible for collecting and generating reports.
yum install sysstat
systemctl start sysstat
systemctl enable sysstat

# show CPU utilization 5 times every 2 seconds.
sar 2 5

# show memory  utilization 5 times every 2 seconds.
sar -r 2 5

# show paging statistics 5 times every 2 seconds.
sar -B 2 5

# To generate all network statistic:
sar -n ALL

# reading SAR log file using -f
sar -f /var/log/sa/sa31|tail
```

##### Reading from journal file
```bash
journalctl --file ./log/journal/a90c18f62af546ccba02fa3734f00a04/system.journal  --since "2020-02-11 00:00:00"
```

##### Show a listing of last logged in users.
```bash
lastb
```

##### Show a listing of current logged in users, print information of them
```bash
who
```
##### Show who is logged on and what they are doing
```bash
w
```

##### Print the user names of users currently logged in to the current host.
```bash
users
```

##### Stop tailing a file on program terminate
```bash
tail -f --pid=<PID> filename.txt
# replace <PID> with the process ID of the program.
```

##### List all enabled services
```bash
systemctl list-unit-files|grep enabled
```


## Hardware
[[back to top](#handy-bash-one-liners)]

##### Collect and summarize all hardware info of your machine
```bash
lshw -json >report.json
# Other options are: [ -html ]  [ -short ]  [ -xml ]  [ -json ]  [ -businfo ]  [ -sanitize ] ,etc
```

##### Finding Out memory device detail
```bash
sudo dmidecode -t memory
```

##### Print detail of CPU hardware
```bash
dmidecode -t 4
#          Type   Information
#          0   BIOS
#          1   System
#          2   Base Board
#          3   Chassis
#          4   Processor
#          5   Memory Controller
#          6   Memory Module
#          7   Cache
#          8   Port Connector
#          9   System Slots
#         11   OEM Strings
#         13   BIOS Language
#         15   System Event Log
#         16   Physical Memory Array
#         17   Memory Device
#         18   32-bit Memory Error
#         19   Memory Array Mapped Address
#         20   Memory Device Mapped Address
#         21   Built-in Pointing Device
#         22   Portable Battery
#         23   System Reset
#         24   Hardware Security
#         25   System Power Controls
#         26   Voltage Probe
#         27   Cooling Device
#         28   Temperature Probe
#         29   Electrical Current Probe
#         30   Out-of-band Remote Access
#         31   Boot Integrity Services
#         32   System Boot
#         34   Management Device
#         35   Management Device Component
#         36   Management Device Threshold Data
#         37   Memory Channel
#         38   IPMI Device
#         39   Power Supply
```
##### Count the number of Segate hard disks
```bash
lsscsi|grep SEAGATE|wc -l
# or
sg_map -i -x|grep SEAGATE|wc -l
```

##### Get UUID of a disk (e.g. sdb)
```bash
lsblk -f /dev/sdb

# or
sudo blkid /dev/sdb
```

##### Generate an UUID
```bash
uuidgen
```

##### Print detail of all hard disks
```bash
lsblk -io KNAME,TYPE,MODEL,VENDOR,SIZE,ROTA
#where ROTA means rotational device / spinning hard disks (1 if true, 0 if false)
```

##### List all PCI (Peripheral Component Interconnect) devices
```bash
lspci
# List information about NIC
lspci | egrep -i --color 'network|ethernet'
```

##### List all USB devices
```bash
lsusb
```

##### Linux modules
```bash
# Show the status of modules in the Linux Kernel
lsmod

# Add and remove modules from the Linux Kernel
modprobe

# or
# Remove a module
rmmod

# Insert a module
insmod
```

##### Controlling IPMI-enabled devices (e.g. BMC)
```bash
# Remotely finding out power status of the server
ipmitool -U <bmc_username> -P <bmc_password> -I lanplus -H <bmc_ip_address> power status

# Remotely switching on server
ipmitool -U <bmc_username> -P <bmc_password> -I lanplus -H <bmc_ip_address> power on

# Turn on panel identify light (default 15s)
ipmitool chassis identify 255

# Found out server sensor temperature
ipmitool sensors |grep -i Temp

# Reset BMC
ipmitool bmc reset cold

# Prnt BMC network
ipmitool lan print 1

# Setting BMC network
ipmitool -I bmc lan set 1 ipaddr 192.168.0.55
ipmitool -I bmc lan set 1 netmask 255.255.255.0
ipmitool -I bmc lan set 1 defgw ipaddr 192.168.0.1
```


## Networking
[[back to top](#handy-bash-one-liners)]

##### Resolve a domain to IP address(es)
```bash
dig +short www.example.net

# or
host www.example.net
```

##### Get DNS TXT record a of domain
```bash
dig -t txt www.example.net

# or
host -t txt www.example.net
```

##### Send a ping with a limited TTL to 10 (TTL: Time-To-Live, which is the maximum number of hops that a packet can travel across the Internet before it gets discarded.)
```bash
ping 192.0.2.1 -t 10
```

##### Print the route packets trace to network host
```bash
traceroute example.net
```

##### Check connection to host (e.g. check connection to port 80 and 22 of example.net)
```bash
nc -vw5 example.net 80
# Connection to example.net 80 port [tcp/http] succeeded!

nc -vw5 example.net 22
# nc: connect to example.net port 22 (tcp) timed out: Operation now in progress
# nc: connect to example.net port 22 (tcp) failed: Network is unreachable
```

##### Nc as a chat tool!
```bash
# From server A:
$ sudo nc -l 80
# then you can connect to the 80 port from another server (e.g. server B):
# e.g. telnet <server A IP address> 80
# then type something in server B
# and you will see the result in server A!
```

##### Check which ports are listening for TCP connections from the network
```bash
#notice that some companies might not like you using nmap
nmap -sT -O localhost

# check port 0-65535
nmap  -p0-65535 localhost
```
##### Check if a host is up and scan for open ports, also skip host discovery.
```bash
#skips checking if the host is alive which may sometimes cause a false positive and stop the scan.
$ nmap example.net -Pn

# Example output:
# Starting Nmap 7.01 ( https://nmap.org ) at 2020-07-18 22:59 CST
# Nmap scan report for example.net (172.217.24.14)
# Host is up (0.013s latency).
# Other addresses for example.net (not scanned): 2404:6800:4008:802::200e
# rDNS record for 172.217.24.14: tsa01s07-in-f14.1e100.net
# Not shown: 998 filtered ports
# PORT    STATE SERVICE
# 80/tcp  open  http
# 443/tcp open  https
#
# Nmap done: 1 IP address (1 host up) scanned in 3.99 seconds
```
##### Scan for open ports and OS and version detection (e.g. scan the domain "scanme.nmap.org")
```bash
$ nmap -A -T4 scanme.nmap.org
# -A to enable OS and version detection, script scanning, and traceroute; -T4 for faster execution
```

##### Look up website information (e.g. name server), searches for an object in a RFC 3912 database.
```bash
whois example.net
```

##### Show the SSL certificate of a domain
```bash
openssl s_client -showcerts -connect www.example.net:443
```

##### Display IP address
```bash
ip a
```

##### Display route table
```bash
ip r
```

##### Display ARP cache (ARP cache displays the MAC addresses of device in the same network that you have connected to)
```bash
ip n
```

##### Add transient IP addres (reset after reboot) (e.g. add 192.168.140.3/24 to device eno16777736)
```bash
ip address add 192.168.140.3/24 dev eno16777736
```

##### Persisting network configuration changes
```bash
sudo vi /etc/sysconfig/network-scripts/ifcfg-enoxxx
# then edit the fields: BOOTPROT, DEVICE, IPADDR, NETMASK, GATEWAY, DNS1 etc
```
##### Refresh NetworkManager
```bash
sudo nmcli c reload
```

##### Restart all interfaces
```bash
sudo systemctl restart network.service
```

##### To view hostname, OS, kernal, architecture at the same time!
```bash
hostnamectl
```

##### Set hostname (set all transient, static, pretty hostname at once)
```bash
hostnamectl set-hostname "mynode"
```

##### Find out the web server (e.g Nginx or Apache) of a website
```bash
curl -I http://example.net/
# HTTP/1.1 200 OK
# Server: nginx
# Date: Thu, 02 Jan 2020 07:01:07 GMT
# Content-Type: text/html
# Content-Length: 1119
# Connection: keep-alive
# Vary: Accept-Encoding
# Last-Modified: Mon, 09 Sep 2019 10:37:49 GMT
# ETag: "xxxxxx"
# Accept-Ranges: bytes
# Vary: Accept-Encoding
```

##### Find out the http status code of a URL
```bash
curl -s -o /dev/null -w "%{http_code}" https://www.example.net
```

##### Unshorten a shortended URL
```bash
curl -s -o /dev/null -w "%{redirect_url}" https://bit.ly/34EFwWC
```

##### Perform network throughput tests
```bash
# server side:
$ sudo iperf -s -p 80

# client side:
iperf -c <server IP address> --parallel 2 -i 1 -t 2 -p 80
```

##### To block port 80 (HTTP server) using iptables.
```bash
sudo iptables -A INPUT -p tcp --dport 80 -j DROP

# only block connection from an IP address
sudo iptables –A INPUT –s <IP> -p tcp –dport 80 –j DROP
```

## Data wrangling
[[back to top](#handy-bash-one-liners)]

##### Print some words that start with a particular string (e.g. words start with 'phy')
```bash
# If file is not specified, the file /usr/share/dict/words is used.
look phy|head -n 10
# phycic
# Phyciodes
# phycite
# Phycitidae
# phycitol
# phyco-
# phycochrom
# phycochromaceae
# phycochromaceous
# phycochrome
```

##### Repeat printing string n times (e.g. print 'hello world' five times)
```bash
printf 'hello world\n%.0s' {1..5}
```
##### Do not echo the trailing newline
```bash
username=`echo -n "bashoneliner"`
```

##### Copy a file to multiple files (e.g copy fileA to file(B-D))
```bash
tee <fileA fileB fileC fileD >/dev/null
```

##### Delete all non-printing characters
```bash
tr -dc '[:print:]' < filename
```

##### Remove newline / nextline
```bash
tr --delete '\n' <input.txt >output.txt
```
##### Replace newline
```bash
tr '\n' ' ' <filename
```

##### To uppercase/lowercase
```bash
tr /a-z/ /A-Z/

```
##### Translate a range of characters (e.g. substitute a-z into a)
```bash
echo 'something' |tr a-z a
# aaaaaaaaa
```

##### Compare two files (e.g. fileA, fileB)

```bash
diff fileA fileB
# a: added; d:delete; c:changed

# or
sdiff fileA fileB
# side-to-side merge of file differences
```

##### Compare two files, strip trailing carriage return/ nextline (e.g. fileA, fileB)
```bash
diff fileA fileB --strip-trailing-cr
```

##### Find common/differing lines
```bash
# having two sorted and uniqed files (for example after running `$ sort -uo fileA fileA` and same for fileB):
# ------
# fileA:
# ------
# joey
# kitten
# piglet
# puppy
# ------
# fileB:
# ------
# calf
# chick
# joey
# puppy
#
# Find lines in both files
comm -12 fileA fileB
# joey
# puppy
#
# Find lines in fileB that are NOT in fileA
comm -13 fileA fileB
# calf
# chick
#
# Find lines in fileA that are NOT in fileB
comm -23 fileA fileB
# kitten
# piglet
```

##### Number a file (e.g. fileA)

```bash
nl fileA

#or
nl -nrz fileA
# add leading zeros

#or
nl -w1 -s ' '
# making it simple, blank separate
```

##### Join two files field by field with tab (default join by the first column of both file, and default separator is space)
```bash
# fileA and fileB should have the same ordering of lines.
join -t '\t' fileA fileB

# Join using specified field (e.g. column 3 of fileA and column 5 of fileB)
join -1 3 -2 5 fileA fileB
```

##### Combine/ paste two or more files into columns (e.g. fileA, fileB, fileC)
```bash
paste fileA fileB fileC
# default tab separate
```

##### Group/combine rows into one row
```bash
# e.g.
# AAAA
# BBBB
# CCCC
# DDDD
cat filename|paste - -
# AAAABBBB
# CCCCDDDD
cat filename|paste - - - -
# AAAABBBBCCCCDDDD
```

##### Fastq to fasta (fastq and fasta are common file formats for bioinformatics sequence data)
```bash
cat file.fastq | paste - - - - | sed 's/^@/>/g'| cut -f1-2 | tr '\t' '\n' >file.fa
```

##### Reverse string
```bash
echo 12345| rev
```

##### Generate sequence 1-10
```bash
seq 10
```

##### Find average of input list/file of integers
```bash
i=`wc -l filename|cut -d ' ' -f1`; cat filename| echo "scale=2;(`paste -sd+`)/"$i|bc
```

##### Generate all combination (e.g. 1,2)
```bash
echo {1,2}{1,2}
# 1 1, 1 2, 2 1, 2 2
```

##### Generate all combination (e.g. A,T,C,G)
```bash
set = {A,T,C,G}
group= 5
for ((i=0; i<$group; i++)); do
    repetition=$set$repetition; done
    bash -c "echo "$repetition""
```

##### Read file content to variable
```bash
foo=$(<test1)
```

##### Echo size of variable
```bash
echo ${#foo}
```

##### Echo a tab
```bash
echo -e ' \t '
```

##### Split file into smaller file
```bash
# Split by line (e.g. 1000 lines/smallfile)
split -d -l 1000 largefile.txt

# Split by byte without breaking lines across files
split -C 10 largefile.txt
```

##### Create a large amount of dummy files (e.g 100000 files, 10 bytes each):
```bash
#1. Create a big file
dd if=/dev/zero of=bigfile bs=1 count=1000000

#2. Split the big file to 100000 10-bytes files
 split -b 10 -a 10 bigfile
```

##### Rename all files (e.g. remove ABC from all .gz files)
```bash
rename 's/ABC//' *.gz
```

##### Remove file extension (e.g remove .gz from filename.gz)
```bash
basename filename.gz .gz

zcat filename.gz> $(basename filename.gz .gz).unpacked
```

##### Add file extension to all file(e.g add .txt)
```bash
rename s/$/.txt/ *
# You can use rename -n s/$/.txt/ * to check the result first, it will only print sth like this:
# rename(a, a.txt)
# rename(b, b.txt)
# rename(c, c.txt)
```

##### Squeeze repeat patterns (e.g. /t/t --> /t)
```bash
tr -s "/t" < filename
```

##### Do not print nextline with echo
```bash
echo -e 'text here \c'
```

##### View first 50 characters of file
```bash
head -c 50 file
```

##### Cut and get last column of a file
```bash
cat file|rev | cut -d/ -f1 | rev
```

##### Add one to variable/increment/ i++ a numeric variable (e.g. $var)
```bash
((var++))
# or
var=$((var+1))

```
##### Cut the last column
```bash
cat filename|rev|cut -f1|rev
```

##### Create or replace a file with contents
```bash
cat >myfile
let me add sth here
# exit with ctrl+d

# or using tee
tee myfile
let me add sth else here
# exit with ctrl+d
```

##### Append to a file with contents
```bash
cat >>myfile
let me add sth here
# exit with ctrl+d

# or using tee
tee -a myfile
let me add sth else here
# exit with ctrl+d
```

##### Clear the contents of a file (e.g. filename)
```bash
>filename
```

##### Append to file (e.g. hihi)
```bash
echo 'hihi' >>filename
```

##### Working with json data
```bash
#install the useful jq package
#sudo apt-get install jq
#e.g. to get all the values of the 'url' key, simply pipe the json to the following jq command(you can use .[]. to select inner json, i.e jq '.[].url')
cat file.json | jq '.url'
```

##### Decimal to Binary (e.g get binary of 5)
```bash
D2B=({0..1}{0..1}{0..1}{0..1}{0..1}{0..1}{0..1}{0..1})
echo -e ${D2B[5]}
#00000101
echo -e ${D2B[255]}
#11111111
```

##### Wrap each input line to fit in specified width (e.g 4 integers per line)
```bash
echo "00110010101110001101" | fold -w4
# 0011
# 0010
# 1011
# 1000
# 1101
```

##### Sort a file by column and keep the original order
```bash
sort -k3,3 -s
```

##### Right align a column (right align the 2nd column)
```bash
cat file.txt|rev|column -t|rev
```

##### To both view and store the output
```bash
echo 'hihihihi' | tee outputfile.txt
# use '-a' with tee to append to file.
```

##### Show non-printing (Ctrl) characters with cat
```bash
cat -v filename
```

##### Convert tab to space
```bash
expand filename
```

##### Convert space to tab
```bash
unexpand filename
```

##### Display file in octal ( you can also use od to display hexadecimal, decimal, etc)
```bash
od filename
```

##### Reverse cat a file
```bash
tac filename
```

##### Reverse the result from `uniq -c`
```bash
while read a b; do yes $b |head -n $a ; done <test.txt
```


## Others
[[back to top](#handy-bash-one-liners)]

##### Describe the format and characteristics of image files.
```bash
identify myimage.png
#myimage.png PNG 1049x747 1049x747+0+0 8-bit sRGB 1.006MB 0.000u 0:00.000
```

##### Bash auto-complete (e.g. show options "now tomorrow never" when you press'tab' after typing "dothis")
[More examples](https://iridakos.com/tutorials/2018/03/01/bash-programmable-completion-tutorial.html)
```bash
complete -W "now tomorrow never" dothis
# ~$ dothis  
# never     now       tomorrow
# press 'tab' again to auto-complete after typing 'n' or 't'
```
##### Displays a calendar
```bash
# print the current month, today will be highlighted.
cal
# October 2019      
# Su Mo Tu We Th Fr Sa  
#    1  2  3  4  5  
# 6  7  8  9 10 11 12  
# 13 14 15 16 17 18 19  
# 20 21 22 23 24 25 26  
# 27 28 29 30 31  

# only display November
cal -m 11
```

##### Convert the hexadecimal MD5 checksum value into its base64-encoded format.
```bash
openssl md5 -binary /path/to/file| base64
# NWbeOpeQbtuY0ATWuUeumw==
```

##### Forces applications to use the default language for output
```bash
export LC_ALL=C

# to revert:
unset LC_ALL
```
##### Encode strings as Base64 strings
```bash
echo test|base64
#dGVzdAo=
```

##### Get parent directory of current directory
```bash
dirname `pwd`
```

##### Read .gz file without extracting

```bash
zmore filename

# or
zless filename
```

##### Run command in background, output error file
```bash
some_commands  &>log &

# or
some_commands 2>log &

# or
some_commands 2>&1| tee logfile

# or
some_commands |& tee logfile

# or
some_commands 2>&1 >>outfile
#0: standard input; 1: standard output; 2: standard error
```

##### Run multiple commands in background
```bash
# run sequentially
(sleep 2; sleep 3) &

# run parallelly
sleep 2 & sleep 3 &
```

##### Run process even when logout (immune to hangups, with output to a non-tty)
```bash
# e.g. Run myscript.sh even when log out.
nohup bash myscript.sh
```

##### Send mail
```bash
echo 'heres the content'| mail -a /path/to/attach_file.txt -s 'mail.subject' me@gmail.com
# use -a flag to set send from (-a "From: some@mail.tld")
```

##### Convert .xls to csv
```bash
xls2csv filename
```

##### Make BEEP sound
```bash
speaker-test -t sine -f 1000 -l1
```

##### Set beep duration
```bash
(speaker-test -t sine -f 1000) & pid=$!;sleep 0.1s;kill -9 $pid
```

##### Editing your history
```bash
history -w
vi ~/.bash_history
history -r

#or
history -d [line_number]
```

##### Interacting with history
```bash
# list 5 previous command (similar to `history |tail -n 5` but wont print the history command itself)
fc -l -5
```

##### Delete current bash command
```bash
Ctrl+U

# or
Ctrl+C

# or
Alt+Shift+#
# to make it to history
```

##### Add something to history (e.g. "addmetohistory")
```bash
# addmetodistory
# just add a "#" before~~
```

##### Get last history/record filename
```bash
head !$
```

##### Clean screen
```bash
clear
# or simply Ctrl+l
```

##### Backup with rsync
```bash
rsync -av filename filename.bak
rsync -av directory directory.bak
rsync -av --ignore_existing directory/ directory.bak
rsync -av --update directory directory.bak

rsync -av directory user@ip_address:/path/to/directory.bak
# skip files that are newer on receiver (i prefer this one!)
```

##### Create a temporary directory and `cd` into it
```bash
cd $(mktemp -d)
# for example, this will create a temporary directory "/tmp/tmp.TivmPLUXFT"
```

##### Make all directories at one time!
```bash
mkdir -p project/{lib/ext,bin,src,doc/{html,info,pdf},demo/stat}
# -p: make parent directory
# this will create:
# project/
# project/bin/
# project/demo/
# project/demo/stat/
# project/doc/
# project/doc/html/
# project/doc/info/
# project/doc/pdf/
# project/lib/
# project/lib/ext/
# project/src/
#
# project/
# ├── bin
# ├── demo
# │   └── stat
# ├── doc
# │   ├── html
# │   ├── info
# │   └── pdf
# ├── lib
# │   └── ext
# └── src
```

##### Run command only if another command returns zero exit status (well done)
```bash
cd tmp/ && tar xvf ~/a.tar
```

##### Run command only if another command returns non-zero exit status (not finish)
```bash
cd tmp/a/b/c ||mkdir -p tmp/a/b/c
```

##### Use backslash "\" to break long command
```bash
cd tmp/a/b/c \
> || \
>mkdir -p tmp/a/b/c
```

##### List file type of file (e.g. /tmp/)
```bash
file /tmp/
# tmp/: directory
```

##### Writing Bash script ('#!'' is called shebang )
```bash
#!/bin/bash
file=${1#*.}
# remove string before a "."
```

##### Python simple HTTP Server
```bash
python -m SimpleHTTPServer
# or when using python3:
python3 -m http.server
```

##### Read user input
```bash
read input
echo $input
```

##### Array
```bash
declare -a array=()

# or
declare array=()

# or associative array
declare -A array=()
```

##### Send a directory
```bash
scp -r directoryname user@ip:/path/to/send
```

##### Fork bomb
```bash
# Don't try this at home!
# It is a function that calls itself twice every call until you run out of system resources.
# A '# ' is added in front for safety reason, remove it when seriously you are testing it.
# :(){:|:&};:
```

##### Use the last argument
```bash
!$
```

##### Check last exit code
```bash
echo $?
```

##### Extract .xz
```
unxz filename.tar.xz
# then
tar -xf filename.tar
```

##### Unzip tar.bz2 file (e.g. file.tar.bz2)
```bash
tar xvfj file.tar.bz2
```

##### Unzip tar.xz file (e.g. file.tar.xz)
```bash
unxz file.tar.xz
tar xopf file.tar
```
##### Extract to a path
```bash
tar xvf -C /path/to/directory filename.gz
```

##### Zip the content of a directory without including the directory itself
```bash
# First cd to the directory, they run:
zip -r -D ../myzipfile .
# you will see the myzipfile.zip in the parent directory (cd ..)
```

##### Output a y/n repeatedly until killed
```bash
# 'y':
yes

# or 'n':
yes n

# or 'anything':
yes anything

# pipe yes to other command
yes | rm -r large_directory
```

##### Create large dummy file of certain size instantly (e.g. 10GiB)
```bash
fallocate -l 10G 10Gigfile
```

##### Create dummy file of certain size (e.g. 200mb)
```bash
dd if=/dev/zero of=/dev/shm/200m bs=1024k count=200
# or
dd if=/dev/zero of=/dev/shm/200m bs=1M count=200

# Standard output:
# 200+0 records in
# 200+0 records out
# 209715200 bytes (210 MB) copied, 0.0955679 s, 2.2 GB/s
```

##### Keep /repeatedly executing the same command (e.g Repeat 'wc -l filename' every 1 second)
```bash
watch -n 1 wc -l filename
```

##### Use Bash Strict Mode
```bash
# These options can make your code safer but, depending on how your pipeline is written, it might be too aggressive 
# or it might not catch the errors that you are interested in

# for reference see https://gist.github.com/mohanpedala/1e2ff5661761d3abd0385e8223e16425
#               and https://mywiki.wooledge.org/BashPitfalls#set_-euo_pipefail

set -o errexit      # exit immediately if a pipeline returns a non-zero status
set -o errtrace     # trap ERR from shell functions, command substitutions, and commands from subshell
set -o nounset      # treat unset variables as an error
set -o pipefail     # pipe will exit with last non-zero status, if applicable
set -Eue -o pipefail  # shorthand for above (pipefail has no short option)
```

##### Print commands and their arguments when execute (e.g. echo `expr 10 + 20 `)
```bash
set -x; echo `expr 10 + 20 `
# or
set -o xtrace; echo `expr 10 + 20 `

# to turn it off..
set +x
```

##### Print some meaningful sentences to you (install fortune first)
```bash
fortune
```

##### Colorful (and useful) version of top (install htop first)
```bash
htop
```

##### Press any key to continue
```bash
read -rsp $'Press any key to continue...\n' -n1 key
```

##### Run sql-like command on files from terminal
```bash
# download:
# https://github.com/harelba/q
# example:
q -d "," "select c3,c4,c5 from /path/to/file.txt where c3='foo' and c5='boo'"
```

##### Using Screen for multiple terminal sessions
```bash
# Create session and attach:
screen

# Create a screen and name it 'test'
screen -S test

# Create detached session foo:
screen -S foo -d -m

# Detached session foo:
screen: ^a^d

# List sessions:
screen -ls

# Attach last session:
screen -r

# Attach to session foo:
screen -r foo

# Kill session foo:
screen -r foo -X quit


# Scroll:
# Hit your screen prefix combination (C-a / control+A), then hit Escape.
# Move up/down with the arrow keys (↑ and ↓).  

# Redirect output of an already running process in Screen:
# (C-a / control+A), then hit 'H'  

# Store screen output for Screen:
# Ctrl+A, Shift+H  
# You will then find a screen.log file under current directory.  
```

##### Using Tmux for multiple terminal sessions
```bash
# Create session and attach:
tmux

# Attach to session foo:
tmux attach -t foo

# Detached session foo:
^bd

# List sessions:
tmux ls

# Attach last session:
tmux attach

# Kill session foo:
tmux kill-session -t foo

# Create detached session foo:
tmux new -s foo -d

# Send command to all panes in tmux:
Ctrl-B
:setw synchronize-panes

# Some tmux pane control commands:
Ctrl-B
#   Panes (splits), Press Ctrl+B, then input the following symbol:
#   %  horizontal split
#   "  vertical split
#   o  swap panes
#   q  show pane numbers
#   x  kill pane
#   space - toggle between layouts

#   Distribute Vertically (rows):
select-layout even-vertical
#   or
Ctrl+b, Alt+2

# Distribute horizontally (columns):
select-layout even-horizontal
#   or
Ctrl+b, Alt+1

# Scroll
Ctrl-b then \[ then you can use your normal navigation keys to scroll around.
Press q to quit scroll mode.
```

##### Pass password to ssh
```bash
sshpass -p mypassword ssh root@10.102.14.88 "df -h"
```

##### Wait for a pid (job) to complete
```bash
wait %1
# or
wait $PID
wait ${!}
#wait ${!} to wait till the last background process ($! is the PID of the last background process)
```

##### Convert pdf to txt
```bash
sudo apt-get install poppler-utils
pdftotext example.pdf example.txt
```

##### List only directory
```bash
ls -d */
```

##### List one file per line.
```bash
ls -1
# or list all, do not ignore entries starting with .
ls -1a
```

##### Capture/record/save terminal output (capture everything you type and output)
```bash
script output.txt
# start using terminal
# to logout the screen session (stop saving the contents), type exit.
```

##### List contents of directories in a tree-like format.
```bash
tree
# go to the directory you want to list, and type tree (sudo apt-get install tree)
# output:
# home/
# └── project
#     ├── 1
#     ├── 2
#     ├── 3
#     ├── 4
#     └── 5
#

# set level directories deep (e.g. level 1)
tree -L 1
# home/
# └── project
```

##### Set up virtualenv(sandbox) for python
```bash
# 1. install virtualenv.
sudo apt-get install virtualenv
# 2. Create a directory (name it .venv or whatever name your want) for your new shiny isolated environment.
virtualenv .venv
# 3. source virtual bin
source .venv/bin/activate
# 4. you can check check if you are now inside a sandbox.
type pip
# 5. Now you can install your pip package, here requirements.txt is simply a txt file containing all the packages you want. (e.g tornado==4.5.3).
pip install -r requirements.txt
# 6. Exit virtual environment
deactivate
```



> More coming!!