Shell tricks

$ git branch

* master

development



$ git checkout development

Switched to branch 'development'

$ git checkout - # Switch to previous

Switched to branch 'master'

$ git checkout -

Switched to branch 'development'

cd

$ pwd

/

$ cd /tmp

$ cd - # Switch to previous

/

$ cd -

/tmp

Get global ip

$ curl ifconfig.co # IPv4

50.110.14.21

$ curl -6 ifconfig.co # IPv6

2010:3f3f:113f:0:ea57:4497:7291:e422

Simple commands

Create a script which calls functions by its` first argument. This is very useful to create simple scripts which could be a wrapper for other commands.

#!/usr/bin/env bash



function do_this () { echo "call do_this function"; }



function do_sth() { echo "call do_sth function" }



case "$1" in

    do_this|do_sth) "$1" ;;

esac

Execute it:

$ ./simple-commands.sh do_this

call do_this function

Loop

Write simple one liner loops if you need to do some batch tasks.

$ for i in {1..10}; do echo "$i"; done



# List disk usage by directories

$ for file in */ .*/ ; do du -sh $file; done

Loop with specified increment each iteration

for i in {1..100..2}; do echo $i; done

1

3

5

7

...

Sequences of letters or numbers

Brace expansion is great for lots of things.

$ touch file{a..c}

$ ls

$ command ls

filea fileb filec

$ touch file-{1..15}

$ ls

file-1	file-10	file-11	file-12	file-13	file-14	file-15	file-2	file-3	file-4	file-5	file-6	file-7	file-8	file-9

$ ls file-{9..12}

file-10	file-11	file-12	file-9

$ printf "%s\n" file-{a..c}{1..3}

file-a1

file-a2

file-a3

file-b1

file-b2

file-b3

file-c1

file-c2

file-c3

(If you give printf more arguments than it expects, it automatically loops.)

Reuse arguments

$ ls /tmp

some_file.txt some_archive.tar.gz

$ cd !$

/tmp

Reuse commands

$ echo "reuse me"

reuse me

$ !!

echo "reuse me"

reuse me

Compare output of two commands

diff <(echo "1 2 4") <(echo "1 2 3 4")

1c1

< 1 2 4

---

 > 1 2 3 4

Fix last command

$ ehco foo bar bar

bash: ehco: command not found

$ ^ehco^echo   

foo bar baz

Accept interactive commands

$ yes | ./interactive-command.sh

Are you sure (y/n)

Accepted

yes: standard output: Broken pipe

The error message is printed because yes gets killed by SIGPIPE signal. This happens if the pipe to ./interactive-command.sh gets closed but yes still wants to write into it.

Ignore error message:

$ yes 2>/dev/null | ./interactive-command.sh

Last exit code

$ ls /tmp

some_file.txt

$ echo $?

0

Easy backup

$ cp file.txt{,.bak}

$ ls -1

file.txt

file.txt.bak

Print to stderr

$ >&2 echo hello

hello

Debugging

Add -xv to your bash scripts, i.e.:

/usr/bin/env bash

set -xv

or /bin/bash -xv script.sh

Useful `readline` tricks

If you use the standard bash readline bindings.

C-a (aka CTRL+A) move cursor to beginning of line

C-e (aka CTRL+E) move cursor to end of line

M-. (aka ALT+.) insert last argument of previous command (like !$, but you can edit it)

Repeat command

Execute a command every two seconds and monitor its` output. This is especially useful for waiting until a deployment or infrastructure provisioning is completed, i.e. on aws.

watch -n2 echo hello `

Substrings

$ a="apple orange"



$ echo ${a#* }

orange

$ echo ${a#*p}

ple orange

$ echo ${a##*p}

le orange



$ echo ${a% *}

apple

$ echo ${a%p*}

ap

$ echo ${a%%p*}

a

The # for finding first occurence from the start and % for the first occurence from the end. * for matching any pattern. For greedy matching ## and %%.

More resources

BashFAQ - A full FAQ of useful stuff

shell-tricksSimple bash tricks which make your life easier.