Bash (Unix shell)

Bash Scripting

Getting Started

Here is an example bash script

#!/bin/bash
# A simple variable example
myvariable=Hello
anothervar=Fred

echo $myvariable $anothervar

You can check your bash scripts using https://www.shellcheck.net/.

Usage

Basic If Statements

https://linuxize.com/post/bash-if-else-statement/

if [[ $VAR -gt 10 ]] then
  echo "The variable is greater than 10."
elif [[ $VAR -eq 10 ]] then
  echo "The variable is equal to 10."
else
  echo "The variable is less than 10."
fi

bash if statements

Some common expressions

# File exists and is a regular file.
[ -f $FILE ]
# File is non-zero size
[ -s $FILE ]
# String has length zero
[ -z $STR ]
# String has length non-zero
[ -n $STR ]

# Check if user is root (https://stackoverflow.com/questions/18215973/how-to-check-if-running-as-root-in-a-bash-script)
if [ "$EUID" -ne 0 ]
  then echo "Please run as root"
  exit
fi

Comparisons

-eq is ==
-gt is >
-ge is >=

Arrays

DRIVES=(
  /media/veracrypt1
  /media/veracrypt2
  /media/veracrypt3
  /media/veracrypt4
  /media/veracrypt5
  /media/veracrypt6
  /media/veracrypt7
  /media/veracrypt8
)
# Add to the array
DRIVES+=(/media/veracrypt9)

for i in "${DRIVES[@]}"
do
        ls $i
done

Notes

"${DRIVES[@]}" means every element will be a new word
"${DRIVES[*]}" will expand the array into a single word

Functions

#!/bin/bash 
function say_hello {
    echo Hello $1
}
say_hello World

Redirects

Redirections

# `ls` is an example command

# Input to stdin
ls < word

# Output stdout to file
ls > file.txt

# Append stdout to file
ls >> file.txt

# Output stderr and stdout to file
ls > file.txt 2>&1
ls &> file.txt # Only works in bash, not sh

Trap

To make a bash script exit on ctrl c:

trap "exit" INT

trap 'kill $(jobs -p)' EXIT

Wait

Add wait at the end of a script to wait for all subprocesses.

Brace Expansion

When you type the following:

echo {red,green,blue}_apple

bash will print out

red_apple green_apple blue_apple

If you want to save this into a variable, you can save it as an array:

APPLES=({red,green,blue}_apple)
echo "${APPLES[@]}"

`--`

Reference

-- explicitly ends non-positional arguments for many commands

alias

You can add aliases to your .bashrc to make commmon commands shorter.

alias kc=kubectl
alias tb="tensorboard --logdir"

dirname

Stack overflow

How to get the source directory of the bash script

#!/bin/bash
DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" >/dev/null 2>&1 && pwd)"
cd "${DIR}" || exit 1

Check if running as sudo

Reference

if [ $EUID -ne 0 ]
  then echo "Please run as root"
  exit
fi

`.bashrc`

Your .bashrc file will be loaded for each terminal.

Presentation of Shell Variable (PS1)

Add the following to show your current working directory.

PS1='\[\e]0;\u@\h: \w\a\]${debian_chroot:+($debian_chroot)}\[\033[01;32m\]\u@\h\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]\n\$ '