Bash Shell Scripting

Bash is a command language interpreter. It is the default command interpreter on most GNU/Linux systems. The name is an acronym for the ‘Bourne-Again SHell’.

Scripting allows for automatic command execution that would otherwise take many manually executed interactive steps.

Bash aliases

• shortcuts to bash commands
• syntax

	alias name="command"

Intro to bash shell scripting

• The check with shell

	echo $SHELL
		OR
	echo $0

• To list all available shells

	>~ cat /etc/shells
	# Pathnames of valid login shells.
	# See shells(5) for details.

	/bin/sh
	/bin/bash
	/usr/bin/git-shell
	/bin/zsh
	/usr/bin/zsh

• Basic Shell Script anatomy

	#!/bin/bash for bash
	# Always starts with a shebang
	# This is a comment
	echo "Hello, World!"

• For python the shebang will be #!/usr/bin/python

Running scripts

• Make the script executable
  • chmod +x script_name
  • chmod 700 script_name
• Run the script with
  • bash script_name.sh (runs without execute permission)
  • ./script_name.sh
  • For python python3 script_name.sh (runs without execute permission)
  • source script_name.sh (execute permission is not required)
  • ./ runs the script in sub shell and calling it runs in current shell
  • when run by calling a program binary, it overrides the shebang

Variables in Bash

• A name for a memory location
• Anatomy : name_of_variable assignment_operator value_of_variable

• os=Linux

  os = Linux is not allowed : no spaces

• if there is a space in variable value, enclose it with ""

 os="Debian Bullseye"

• Bash is a weakly typed, no type declaration
• In bash, numbers are only integers no floating points numbers
  • version=11
• variable names cannot start with a
  • space
  • number
  • Cannot have
    • . in a name
    • @ in a name
• variable names can however have with _
  • This is okay: server_name:”Apache 2.4”

• the value of the variable can be referenced using $variable_name

  • echo $os
  • echo "I am learning $os"
  • Note: While referencing variable in string, make sure to use "" and not ''

    	> echo "I am learning $os"
    	I amlearning Linux
    

• NOTICE THE DIFFERENCE with "" and ''

  	> echo 'I am learning $os'
  	I am learning $os
  

• \$ can be used as an escape character to reference variable names instead of variable values.

  	> echo "The value of \$os is $os"
  	The value of $os is Linux
  

• Use keyword set to list all the shell variables and pipe it to less or grep depending on the use case.
• Use keyword unset to remove a variable
• good practice to keep user defined variables names in lower case letters so there is no conflict with env variables
• use declare keyword with -r flag to create a read only variable

  	declare -r logdir="/var/log"
  

  • use case

      ls $logdir
    

  • reassignment is not allowed

      > logdir=abc
      bash: logdir: readonly variable
    

Environment Variables

Each time we launch a terminal window and a shell inside it, a collection of predefined variables are set with dynamic values which are used to customize how a system works. There are two class of such variables

• Environment Variables
  • defined for the current shell
  • inherited by a child shells or processes
  • used to pass information to processes
  • displayed using env or print env
  • Some built in environment variables
    • $PATH : path variable
    • $USER : Current logged in user
    • $HOME : Home directory of $USER
    • $HISTFILE : Shell command history file
    • $HISTSIZE : Max length of $HISTFILE
  • The names are UPPERCASE
  • To create an new env variable, export it in a startup config like .bashrc
  • To and to path: export PATH=$PATH:/path_to_dir/
  • To add a new: export MYVAR=value

• Shell Variables

  • contained exclusively within the shell in which they were set or defined
  • displayed using set
• Configuration
  • user: .bashrc
  • system-wide: /etc/bash.bashrc and /etc/profile

Getting user Input

• Keyword read: read from standard input into shell variables

	> read name
	enter_name_here

• A variable named name is created with the input value
• use -p flag to echo a prompt or hint

	> read -p "Enter your name please: " name
	enter_name_here

• Access name by $name like echo $name
• use -s to hide the user input, for example a password.

Special Variables & Positional Arguments

• Context

  • In apt install neofetch, install is the first argument in the script and neofetch is the second argument.
  • They are separated by a space referred to as inter field separators.

• Example: In ./script.sh filename1 dir1 some_value other_value

  • $0 is the name of the script : script.sh
  • $1 is the first positional argument : filename1
  • $2 is the second positional argument : dir1
  • $4 is the last positional argument : other_value
  • $9 would be the 9th positional argument and so on
  • $# is the number of positional arguments
  • ”$*“ is the string representation of all positional arguments
  • $? is the most recent foreground command exit status

Flow Control

Logic Statements

If, Elif and Else Statements

• Anatomy of an if, elif, else statement
  • NOTE: double [[ ]] is a newer safer way to do test conditions as it works well with strings with spaces and allows for regex matching. But it also works with []

	if [[ this_condition_is_true ]]
	then
		execute_this_code
	elif [[ some_ther_condition_is_true ]]
	then
		execute_this_code
	else
		execute_this_code
	fi

• The man page for test contains all the test condition allowed in an if statement
• check the man page for more.
• Some common conditions
  • -f is a file
  • -d is a directory

Testing conditions for numbers

• Some common testing conditions for numbers

  INTEGER1 -eq INTEGER2
    INTEGER1 is equal to INTEGER2

  INTEGER1 -ge INTEGER2
    INTEGER1 is greater than or equal to INTEGER2

  INTEGER1 -gt INTEGER2
    INTEGER1 is greater than INTEGER2

  INTEGER1 -le INTEGER2
    INTEGER1 is less than or equal to INTEGER2

  INTEGER1 -lt INTEGER2
    INTEGER1 is less than INTEGER2

  INTEGER1 -ne INTEGER2
    INTEGER1 is not equal to INTEGER2

• Example

	#!/bin/bash
	read -p "Enter and number: " guess
	if [[ $guess -eq 5 ]]
	then
		echo "Jackpot"
	else
		echo "Okay looser!"
	fi

• Some testing operations for strings

  -n STRING
    the length of STRING is nonzero

  STRING equivalent to -n STRING

  -z STRING
    the length of STRING is zero

  STRING1 = STRING2
    the strings are equal

  STRING1 != STRING2
    the strings are not equal

• Example

	#!/bin/bash
	read -p "String1: " str1
	read -p "String2: " str2

	########################
	# Method 1
	########################
	if [ "$str1" = "$str2" ]
	then
		echo "Equal strings"
	else
		echo "Not Equal strings"
	if
	########################
	########################
	# Method 2
	########################
	if [[ "$str1" == "$str2" ]]
	then
		echo "Equal strings"
	else
		echo "Not Equal strings"
	if
	########################

• Example: Sub strings

	str1="some strings with linux in it."

	if [[ '$str1' == *"linux"* ]]
	then
		echo "The substring linux IS present."
	else
		echo "The substring linux IS NOT present."
	fi

Multiple testing conditions and nested if statements

• For multiple conditions

  • the logical or and and operators can be used
    • or is ||
    • and is &&
  • syntax
  • [[ condition 1 ]] && [[ condition 2 ]]
  • [[ condition 1 ]] || [[ condition 2 ]]
  • Alternatively

  EXPRESSION1 -a EXPRESSION2
        both EXPRESSION1 and EXPRESSION2 are true
  
  EXPRESSION1 -o EXPRESSION2
        either EXPRESSION1 or EXPRESSION2 is true
  

• Anatomy of a nested if statement

	if [[ condition ]]
	then
		if [[ condition 2]]
		then
			do_this
		else
			do_this
	else
		do_this
	fi

For loop (Loops)

• Anatomy of a for loop

	for item in LIST
	  do
	    COMMANDS
	done

• Example: Iterate through a list

	#!/bin/bash
	for os in Ubuntu CentOS Slackware Kali "MX Linux"
	do
	  echo "OS is $os"
	done

• Example: Iterate through a range of numbers
  • Specify range {start_num..end_num}
  • with step {start_num..end_num..step

	for num in {3..7}
	do
	  echo "num is $num"
	done

• Example: Loop through items in a directory.

	for item in ./*
	do
	  if [[ -f "$item" ]]
	  then
		echo "$item"
	  fi
	done

While loop (Loops)

• Anatomy of a while loop.
• It runs as long as a specific command is true

	while CONDITION_IS_TRUE
	do
		COMMANDS
	done

• Example: Iterate through numbers

	i=0

	while [[ $i -lt 10]]
	do
		echo "i: $i"
		((i++))
	done

• Example: Infinite loop

	#!/bin/bash
	while true
	do
	  echo "In an infinite loop, press CTRL+C to exit."
	  sleep 1
	done

• alternates
  • while [[ true ]]
  • while :
  • while true; do echo "infinite loop"; sleep 1; done

Command Substitutions

• To store the output of a command into a variable

	# Method 1
	var_one=`command`

	# Method 2
	var_two="$(command)"

• For arithmetic operations use
  • ((arithmetic_operation))

Case Statements

• Its a simpler alternative to if,elif,else
• It is similar to switch statement in C, C++ & Java
• Anatomy of the a case statement
  • PATTERN can contain special characters
  • STATEMENT blocks end with ;; (double semicolon)
  • * PATTERN is a default case

	case EXPRESSION in
		PATTERN_1)
			STATEMENTS
			;;
		PATTERN_2)
			STATEMENTS
			;;
		PATTERN_3)
			STATEMENTS
			;;
		PATTERN_4)
			STATEMENTS
			;;
		""
		""
		*)
			STATEMENTS
			;;
	esac

• Example
  • Multiple options in case option_1|option_2
  • Enclose "A String With Spaces" inside ""

	#!/bin/bash

	echo -n "Enter your favorite pet: "
	read PET

	case "$PET" in
		dog)
			echo "Your favorite pet is the dog."
			;;
		cat|Cat)
			echo "You like cats."
			;;
		fish|"African Turtle")
			echo "Fish or turtles are great."
			;;
		*)
			echo "Your favorite pet is weird"
	esac

• Example: Managing processes signal.sh

	#!/bin/bash
	if [[ $# -ne 2 ]]
	then
	  echo "Run the script with 2 arguments: Signal and PID."
	  exit
	fi

	case "$1" in
	  1)
		echo "Sending the SIGHUP signal to $2"
		kill -SIGHUP $2
		;;

	  2)
		echo "Sending the SIGINT signal to $2"
		kill -SIGINT $2
		;;
	  15)
		echo "Sending the SIGTERM signal to $2"
		kill -15 $2
		;;
	  *)
		echo "Signal number $1 will not be delivered"
	esac

Functions in Bash

• Used to organize code
  • modularity
  • reusability
• Compared to other programming languages, bash functions are limited.
• Anatomy of a bash function
  • the () in functions definitions are purely decorative
    • they do not take any arguments in them
  • the arguments can be passed into functions similar to command line arguments, i.e., refer to the variable or arguments with $varibale_name or $1 $2 etc.
  • bash functions do not allow a return value

	#!/bin/bash
	function name_of_function () {
		one thing to do
		two thing to do
	}

	OR

	name_of_function () {
		do_something
	}

	# This is a function call
	name_of_function

Variable scope in function

• By default in bash all variables are global
• To declare a local variable inside a function, use the keyword local

	var1-"AA"
	var2="BB"

	func1() {
		var1="XX"
		local var2="YY"
		echo "Inside func1: var1=$var1, var2=$var2"
	}

	func1
	echo "Outside func1: var1=$var1, var2=$var2"

• Output from above

	Inside func1: var1=XX, var2=YY
	Outside func1: var1=XX, var2=BB

Menus in bash; The Select Statement

• Anatomy of a select statement
  • ITEM is a user defined variable
  • LIST is a series of strings or output of commands
    • if a "string contains a space" enclose it in ""
  • The selection is stored in a predefined variable REPLY
  • The default prompt is #? and can be change by setting a predefined variable PS3

	select ITEM in LIST
	do
		COMMANDS
	done

• Example

	#!/bin/bash
	PS3="Choose your country"
	select COUNTRY in India Germany France USA "United Kingdom" Quit
	do
	  case $REPLY in
		1)
		  echo "You speak many languages"
		  ;;
		2)
		  echo "You speak many languages"
		  ;;
		3)
		  echo "You speak many languages"
		  ;;
		4)
		  echo "You just speak amarican english"
		  ;;
		5)
		  echo "You speak many languages"
		6)
		  echo "Qutting..."
		  break
		  ;;
		*)
		  echo "Invalid Option"
	done

Scripts for reference

Github Repo: bass-scripting-basics