Shell Scripting

A shell script is a computer program designed to be run by a Unix shell, a command-line interpreter. The various dialects of shell scripts are considered to be scripting languages. Typical operations performed by shell scripts include file manipulation, program execution, and printing text.

The program that interprets and executes the various commands that we type in the terminal
Translates our command that the OS Kernel can understand

This wiki is intended to hold documentation of GNU Bash. The main motivation was to provide human-readable documentation and information. the docs here are not meant as a newbie tutorial.

Shebang #!

There is a line at the beginning of scripts which starts with #! and continues with an executable who needs to run this script. It is called shebang and as mentioned, tells the shell which interpreter must be used to run this script.

#!/bin/bash

Note: In many programming languages (including Bash & Python), a # at the beginning of a line in script indicates comments. Do not confuse it with the Shebang (#!)

#!/bin/bash

echo

echo "We are learning! Wowww..."

echo

Note: You should make your script executable first:*chmod +x script.sh*

to run an script: ./script.sh

Variables

Already seen in the previous section. You can define variables like this VARNAME=VALUE. Here is a sample:

#!/bin/bash

NAME=Mostafa

echo

echo "$NAME is learning! Wowww..." 

echo

In bash you can access your variable via $

Note: you can also do NAME="Mostafa is awesome" if you need to have spaces in your values

If you want to access the command line arguments in your shell script, use $1, $2, .... You can find the number of command line arguments via $# variable.

Store output of a command in a variable:

variable_name=$(command)

variable_name=`command`

Bash syntax and operations

Compound commands overview


Grouping		Loops
( … )	command grouping in a subshell	for word in …; do …; done	classic for-loop
{ …; }	command grouping	for ((x=1; x<=10; x++)); do ...; done	C-style for-loop
Conditionals		while …; do …; done	while loop
[[ ... ]]	conditional expression	until …; do …; done	until loop
if …; then …; fi	conditional branching	Misc
case … esac	pattern-based branching	(( ... ))	arithmetic evaluation
		select word in …; do …; done	user selections

Performing Math

Use expr or $[ ] or $(( )) to do math in Bash.


Operator
expr ARG1 + ARG2	expr ARG1 > ARG2
expr ARG1 - ARG2	expr ARG1 >= ARG2
*expr ARG1 ARG2**	expr ARG1 != ARG2
expr ARG1 / ARG2	expr ARG1 = ARG2
expr ARG1 % ARG2	expr ARG1 <= ARG2
expr ARG1 < ARG2

#!/bin/bash
a=10
b=20


echo `expr $a - $b`
echo `expr $a + $b`
echo $(($a - $b))
echo $[ $a + $b ]
----------
./bash.sh
-10
30
-10
30

Bash Built-in Variables


Variables	Description
$?	Exit status of last recently executed command (0 or 1).
$0	Script name.
$1 ~ 9	n^th^ argument passed to the script or function.
$*	a single string that consists of all of the positional parameters, separated by the first character in the environment variable IFS (internal field separator), which is a space, TAB, and NEWLINE by default.
$@	All arguments passed to the script or function; each argument is a separate word.
$#	Number of arguments passed to the script or function.
$$	PID of the script in which this variable is called.
$!	PID of the last recently executed background command.
$_	set to last argument of previous command executed.
$RANDOM	Pseudorandom integer value between 0 to 32767.

$? Exit Status Codes


Code	Description
0	Successful completion of the command.
1	General unknown error.
2	Misuse of shell command.
126	The command can’t execute.
127	Command not found.
128	Invalid exit argument.
128+x	Fatal error with Linux signal x.
130	Command terminated with Ctrl+C.
255	Exit status out of range.

Note: Unlike any other programing langues 0 is True and 1 is False.

Condition


Expression	Meaning	Operator
STRING1 = STRING2	Strings are equal.	=
STRING1 != STRING2	Strings are not equal.	≠
-n STRING1	String1 has a length greater than zero.
-z STRING1	String1 has a length of zero.
INT1 -eq INT2	INT1 is equal to INT2.	=
INT1 -ne INT2	INT1 is not equal to INT2.	≠
INT1 -ge INT2	INT1 is greater than or equal to INT2.	≥
INT1 -gt INT2	INT1 is greater than INT2.	>
INT1 -le INT2	INT1 is less than or equal to INT2.	≤
INT1 -lt INT2	INT1 is less than INT2.	<
File1 -nt File2	File1 is newer than File2
File1 -ot File2	File1 is older than File2

Using && and || in Bash Scripts

`&&` (AND) and `||` (OR)

Operator	Description	Example
!	This is logical negation. This inverts a true condition into false and vice versa.	[ ! false ] is true.
-o	This is logical OR. If one of the operands is true, then the condition becomes true.	[ $a -lt 20 -o $b -gt 100 ] is true.
-a	This is logical AND. If both the operands are true, then the condition becomes true otherwise false.	[ $a -lt 20 -a $b -gt 100 ] is false

#!/bin/bash

a=10
b=20

[ $a -eq 10 -o $b -eq 20 ]
echo $?
[ $a -eq 10 -a $b -gt 20 ]
echo $?
[ $a -eq 10 ] || [ $b -ge 10 ]
echo $? 
[ $a -eq 10 ] && [ $b -ge 10 ]
echo $?
[[ $a -eq 10 || $b -ge 10 ]]
echo $?
[[ $a -eq 10 && $b -eq 10 ]]
echo $?
-------
./bash.sh 
0
1
0
0
0
1

Functions

Creating Functions

To declare a function, simply use the following syntax −

function_name () { 
   list of commands
}

The name of your function is function_name, and that's what you will use to call it from elsewhere in your scripts. The function name must be followed by parentheses, followed by a list of commands enclosed within braces.

Example

Following example shows the use of function −

#!/bin/sh

# Define your function here
Hello () {
   echo "Hello World"
}

# Invoke your function
Hello

Upon execution, you will receive the following output −

$./test.sh Hello World

Pass Parameters to a Function

You can define a function that will accept parameters while calling the function. These parameters would be represented by $1, $2 and so on.

Following is an example where we pass two parameters Zara and Ali and then we capture and print these parameters in the function.

#!/bin/sh

# Define your function here
Hello () {
   echo "Hello World $1 $2"
}

# Invoke your function
Hello Mostafa Mohammad
---------------
$./test.sh
Hello World Mostafa Mohammad

Returning Values from Functions

If you execute an exit command from inside a function, its effect is not only to terminate execution of the function but also of the shell program that called the function.

If you instead want to just terminate execution of the function, then there is way to come out of a defined function.

Based on the situation you can return any value from your function using the return command whose syntax is as follows −

return code

Here code can be anything you choose here, but obviously you should choose something that is meaningful or useful in the context of your script as a whole.

Example

Following function returns a value 10 −

#!/bin/sh

# Define your function here
Hello () {
   echo "Hello World $1 $2"
   return 10
}

# Invoke your function
Hello Zara Ali

# Capture value returnd by last command
ret=$?

echo "Return value is $ret"

Upon execution, you will receive the following result −

$./test.sh Hello World Mostafa Mohammad Return value is 10

Nested Functions

One of the more interesting features of functions is that they can call themselves and also other functions. A function that calls itself is known as a recursive function.

Following example demonstrates nesting of two functions −

#!/bin/sh

# Calling one function from another
number_one () {
   echo "This is the first function speaking..."
   number_two
}

number_two () {
   echo "This is now the second function speaking..."
}

# Calling function one.
number_one
------------------------
This is the first function speaking...
This is now the second function speaking...

Function Call from Prompt

You can put definitions for commonly used functions inside your .profile. These definitions will be available whenever you log in and you can use them at the command prompt.

Alternatively, you can group the definitions in a file, say test.sh, and then execute the file in the current shell by typing −

$. test.sh

This has the effect of causing functions defined inside test.sh to be read and defined to the current shell as follows −

$ number_one This is the first function speaking... This is now the second function speaking... $

To remove the definition of a function from the shell, use the unset command with the .f option. This command is also used to remove the definition of a variable to the shell.

$ unset -f function_name

Name		Name	Last commit message	Last commit date
Latest commit History 16 Commits
Exercises		Exercises
service_checker		service_checker
README.md		README.md

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Shell Scripting

Shebang #!

Variables

Bash syntax and operations

Compound commands overview

Performing Math

Bash Built-in Variables

$? Exit Status Codes

Condition

Using && and || in Bash Scripts

`&&` (AND) and `||` (OR)

Functions

Creating Functions

Example

Pass Parameters to a Function

Returning Values from Functions

Example

Nested Functions

Function Call from Prompt

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Shell Scripting

Shebang #!

Variables

Bash syntax and operations

Compound commands overview

Performing Math

Bash Built-in Variables

$? Exit Status Codes

Condition

Using && and || in Bash Scripts

&& (AND) and || (OR)

Functions

Creating Functions

Example

Pass Parameters to a Function

Returning Values from Functions

Example

Nested Functions

Function Call from Prompt

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