fix command #last and add new commands for #System and #Others

Make formatting and names more consistent

README.md

@@ -545,7 +545,7 @@ sed -i '$ s/.$//' filename
 
 ##### Add string to beginning of file (e.g. "\[")
 ```bash
-sed -i '1s/^/[/' file
+sed -i '1s/^/[/' filename
 ```
 
 ##### Add string at certain line number (e.g. add 'something' to line 1 and line 3)
@@ -692,7 +692,7 @@ sed '$ s/.$//'
 
 ##### Insert character at specified position of file (e.g. AAAAAA --> AAA#AAA)
 ```bash
-sed -r -e 's/^.{3}/&#/' file
+sed -r -e 's/^.{3}/&#/' filename
 ```
 
 
@@ -747,7 +747,7 @@ 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
+ls|xargs -n1 -I file awk '{s=$0};END{print FILENAME,s}' filename
 ```
 
 ##### Add string to the beginning of a column (e.g add "chr" to column $3)
@@ -757,12 +757,12 @@ awk 'BEGIN{OFS="\t"}$3="chr"$3'
 
 ##### Remove lines with string (e.g. 'bbo')
 ```bash
-awk '!/bbo/' file
+awk '!/bbo/' filename
 ```
 
 ##### Remove last column
 ```bash
-awk 'NF{NF-=1};1' file
+awk 'NF{NF-=1};1' filename
 ```
 
 ##### Usage and meaning of NR and FNR
@@ -827,7 +827,7 @@ awk '{printf("%s\t%s\n",NR,$0)}'
 # David    cat
 # David    dog
 
-awk '{split($2,a,",");for(i in a)print $1"\t"a[i]}' file
+awk '{split($2,a,",");for(i in a)print $1"\t"a[i]}' filename
 
 # Detail here:　http://stackoverflow.com/questions/33408762/bash-turning-single-comma-separated-column-into-multi-line-string
 ```
@@ -941,7 +941,7 @@ find /dir/to/A -type f -name "*.py" -print 0| xargs -0 -r -I file cp -v -p file
 
 ##### With sed
 ```bash
-ls |xargs -n1 -I file sed -i '/^Pos/d' file
+ls |xargs -n1 -I file sed -i '/^Pos/d' filename
 ```
 
 ##### Add the file name to the first line of file
@@ -1052,9 +1052,10 @@ else
   echo >&2 "Fatal error. This script requires mydir."
 fi
 
-# if variable is null
+# Check if a variable is null
-if [ ! -s "myvariable" ]; then echo -e "variable is null!" ; fi
+if [ -z "$var" ]; then echo "NULL"; else echo "Not NULL"; fi
-#True of the length if "STRING" is zero.
+# or 
+[ -z "$var" ] && echo "NULL"
 
 # 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
@@ -1551,9 +1552,17 @@ stat filename.txt
 ps aux
 ```
 
+##### List processes by top memory usage
+```bash
+ps -eo pid,ppid,cmd,%mem,%cpu --sort=-%mem | head
+```
+
 ##### Display a tree of processes
 ```bash
 pstree
+
+# or
+ps aux --forest
 ```
 
 ##### Find maximum number of processes
@@ -1959,7 +1968,7 @@ ldconfig -p
 ldd /bin/ls
 ```
 
-##### Check user login
+##### Check the most recent login of all users
 ```bash
 lastlog
 ```
@@ -2272,7 +2281,12 @@ sar -f /var/log/sa/sa31|tail
 journalctl --file ./log/journal/a90c18f62af546ccba02fa3734f00a04/system.journal  --since "2020-02-11 00:00:00"
 ```
 
-##### Show a listing of last logged in users.
+##### Show a listing of last logged in users
+```bash
+last
+```
+
+##### Show a listing of unsuccessful (bad) login attempts 
 ```bash
 lastb
 ```
@@ -2450,6 +2464,11 @@ dig +short www.example.com
 host www.example.com
 ```
 
+##### Check public IP address
+```bash
+curl http://checkip.amazonaws.com
+```
+
 ##### Get DNS TXT record a of domain
 ```bash
 dig -t txt www.example.com
@@ -2490,7 +2509,7 @@ $ sudo nc -l 80
 
 ##### Check which ports are listening for TCP connections from the network
 ```bash
-#notice that some companies might not like you using nmap
+# note that some companies might not like you using nmap
 nmap -sT -O localhost
 
 # check port 0-65535
@@ -2498,7 +2517,7 @@ 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.
+# skips checking if the host is alive. this may sometimes cause a false positive, stopping the scan.
 $ nmap google.com -Pn
 
 # Example output:
@@ -2520,7 +2539,7 @@ $ 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.
+##### Look up website information (e.g. name server), searches for an object in a RFC 3912 database
 ```bash
 whois google.com
 ```
@@ -2530,7 +2549,7 @@ whois google.com
 openssl s_client -showcerts -connect www.example.com:443
 ```
 
-##### Display IP address
+##### Display network interfaces and their associated IP addresses
 ```bash
 ip a
 ```
@@ -2590,6 +2609,10 @@ curl -I http://example.com/
 # Accept-Ranges: bytes
 # Vary: Accept-Encoding
 ```
+##### Find out the time spent between request and response
+```
+curl -v -o /dev/null -s -w 'Total: %{time_total}s\n' google.com
+```
 
 ##### Find out the http status code of a URL
 ```bash
@@ -2870,7 +2893,7 @@ echo -e 'text here \c'
 
 ##### View first 50 characters of file
 ```bash
-head -c 50 file
+head -c 50 filename
 ```
 
 ##### Cut and get last column of a file
@@ -2908,6 +2931,11 @@ cat >>myfile
 let me add sth here
 # exit with ctrl+d
 
+# or
+cat << EoF >> filename
+> add something here
+> EoF 
+
 # or using tee
 tee -a myfile
 let me add sth else here
@@ -2926,7 +2954,7 @@ echo 'hihi' >>filename
 
 ##### Working with json data
 ```bash
-#install the useful jq package
+# 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'
@@ -3001,7 +3029,7 @@ 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.
+##### 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
@@ -3277,6 +3305,12 @@ scp -r directoryname user@ip:/path/to/send
 # :(){:|:&};:
 ```
 
+##### Trigger kernel crash
+```bash
+# Don't try this at home! 
+echo c > /proc/sysrq-trigger
+```
+
 ##### Use the last argument
 ```bash
 !$