Bash is the de-factor command line shell for linux. While not the most intuitive, it can be extremely terse yet powetful. Anyone who uses linux should have knowledge of at least the basics of bash.

#!/bin/bash
echo "Hello world"

$ bash ./script.sh

# make the script executable
$ chmod +x ./script.sh

# run directly
$ ./script.sh

Variables

NAME="Moeen"
MESSAGE="Hello world"

echo $MESSAGE
echo "My name ${NAME}"

• It is a convention that all variables should be uppercase
• There shouldn’t be a space before or after the equals (i.e. =) sign when declaring variables
• Variables can be interpolated in strings. This only works in double-quote strings

Arrays

declare -a packages=(
  "libx11-dev"
  "libfontconfig1-dev"
  "libxft-dev"
)

# loop through the array
for pkg in ${packages[@]}; do
  echo $pkg
done

# concatenate array values
packages_str=""
for pkg in ${packages[@]}; do
  packages_str="${packages_str}${pkg} "
done

echo $packages_str

User Input

Interactive input

read -p "Enter name: " NAME
echo "Your name is ${NAME}"

Command-line Arguments

# print all args including script name
echo $0 $1 $2

# print all args excluding script name
echo $@

Capture output of command

NAME=$(whoami)

Control Flow

NAME="User"

if [[ $NAME == "User" ]]; then
  echo "You are ${NAME}"
elif [[ $NAME == "Admin" ]]; then
  echo "You are z -> ${NAME}"
else
  echo "You are unknown"
fi

Value Comparison

A=30
B=40

# -eq	(==)	values are equal
# -ne	(!=)	values are not equal
# -gt		first value greater than second value
# -ge		first value greater or equal to second value
# -lt		first value less than second value
# -le		first value less than or equal to second value
if [[ $A -eq $B ]]; then
  echo "values are equal"
fi

File Checks

FILENAME="example.txt"

# -e		check if exists
# -d 		check if directory 
# -f 		check if file
# -r 		check if file is readable
# -w 		check if file is writable
# -x 		check if file is executable

if [[ -e $FILENAME ]]; then
  echo "file exists"
fi

Execute Commands

update_cmd="sudo apt-get update && sudo apt-get upgrade -y"
eval $update_cmd

Function

function greet() {
  name=$1
  echo "Hello, $name!"
}

greet 'admin'

function combine() {
  input=$@
  result=""
  for entry in $input; do
    result+="$entry, "
  done

  echo $result
}

declare -a roles=('admin' 'customer' 'another')
combine "${roles[@]}"