Shell tricks

• Switch to previous directory
  • git
  • cd
• Get global ip
• Simple commands
• Loop
• Loop with specified increment each iteration
• Sequences of letters or numbers
• Reuse arguments
• Reuse commands
• Compare output of two commands
• Fix last command
• Accept interactive commands
• Last exit code
• Easy backup
• Print to stderr
• Debugging
• Useful readline tricks
• Repeat command
• Substrings
• More resources

Switch to previous directory

Switch between the current and previous branch / directory.

git

$ 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