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@ -1145,29 +1145,6 @@ date --date @1615852800
``` ```
##### 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) ##### wait for random duration (e.g. sleep 1-5 second, like adding a jitter)
```bash ```bash
sleep $[ ( $RANDOM % 5 ) + 1 ] sleep $[ ( $RANDOM % 5 ) + 1 ]
@ -2860,28 +2837,12 @@ var=$((var+1))
cat filename|rev|cut -f1|rev cat filename|rev|cut -f1|rev
``` ```
##### Create or replace a file with contents ##### Cat to a file
```bash ```bash
cat >myfile cat >myfile
let me add sth here let me add sth here
# exit with ctrl+d exit by control + c
^C
# 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) ##### Clear the contents of a file (e.g. filename)
@ -3279,9 +3240,9 @@ fallocate -l 10G 10Gigfile
##### Create dummy file of certain size (e.g. 200mb) ##### Create dummy file of certain size (e.g. 200mb)
```bash ```bash
dd if=/dev/zero of=/dev/shm/200m bs=1024k count=200 dd if=/dev/zero of=//dev/shm/200m bs=1024k count=200
# or # or
dd if=/dev/zero of=/dev/shm/200m bs=1M count=200 dd if=/dev/zero of=//dev/shm/200m bs=1M count=200
# Standard output: # Standard output:
# 200+0 records in # 200+0 records in
@ -3294,29 +3255,9 @@ dd if=/dev/zero of=/dev/shm/200m bs=1M count=200
watch -n 1 wc -l filename 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 `) ##### Print commands and their arguments when execute (e.g. echo `expr 10 + 20 `)
```bash ```bash
set -x; echo `expr 10 + 20 ` 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) ##### Print some meaningful sentences to you (install fortune first)