Shell scripting - Logical structures loops and conditions

Aerosol · December 8, 2014

Like many other modern high level programing languages or code interpreters, Linux shell has capability to use conditional statements. Often when writing a computer code, theres a need to perform different actions based on various logical decisions. For this purpose, it is possible to use conditional statements.

Syntax of If/else statement:

if condition_is_true
then execute commands
else
execute commands
fi

Example below demonstrates simple if/else statement structure. If condition is satisfied, instructions after "then" syntax will be procesed, otherwise code or commands after "else" statement are executed:

LinuxBox#vi my_script.sh
 #!/bin/bash
 # Condition check if/else statement

 number=5

 if no=5; then
        echo "Number is 5 !"
 else
        echo "Number is not 5 !"
 fi


 :wq!

 LinuxBox#
 LinuxBox# ./my_script.sh
 Number is 5 !

We can use simple If/else statement for example in a script that checks if directory exists:

LinuxBox# vi my_script.sh
 #!/bin/bash
 # simple if/else directory existence check

 directory="./myScripts"

 #check if directory exists
 if [ -d $directory ]; then
	echo "Directory exists !"
 else 
	echo "Directory does not exists !"
 fi 


 LinuxBox# ./my_script.sh
 Directory does not exists !
 LinuxBox# mkdir myScripts
 LinuxBox# ./my_script.sh
 Directory exists !

While structure (loops)

The while construction allows for repetitive execution of code or list of commands, as long as the command controlling the while loop executes successfully (exit status of zero).

Syntax of While structure:

while condition_is_true
do
execute these commands
done

This example shows while syntax. While loop keeps looping as long as counter is greater than 0:

LinuxBox# vi my_script.sh
 #!/bin/bash
 # simple while looop script
 COUNT=5 
 while [ $COUNT -gt 0 ]; do
	echo Value of counter is: $COUNT
	let COUNT=COUNT-1
 done 

 LinuxBox# ./my_script.sh
 Value of counter is: 5
 Value of counter is: 4
 Value of counter is: 3
 Value of counter is: 2
 Value of counter is: 1

(Loop use Relational Operators in "[while condition]" to check for how long script has to loop. To see more about Relational Operators check this tutorial )

This example use while loop to get the right user input. Until user selects the right answer, script will keep (looping) repeating itself:

 LinuxBox# vi my_script.sh 
 #!/bin/bash
 # while loop 
 # Declare variable choice and assign value 0
 choice=0
 # Print to stdout
 echo "1. Shell"
 echo "2. Scripting"
 echo "3. ITTutorials"
 echo -n "Please enter a choice [1,2 or 3]? "
 # Loop while the variable choice is equal 0
 while [ $choice -eq 0 ]; do 
 # read user input
 read choice 
 if [ $choice -eq 1 ]; then 
        echo "Your choice is: Bash"
 else                   
 if [ $choice -eq 2 ] ; then
    echo "Your choice is: Scripting"
 else
 if [ $choice -eq 3 ] ; then
    echo "Your choice is: ITTutorials"
 else
    # user haven't entered the right choice so we'll ask the question again
    echo "Please make a choice between 1-3 !"
    echo "1. Bash"
    echo "2. Scripting"
    echo "3. ITTutorials"
    echo -n "Please choose a word [1,2 or 3]? "
	# and reset choice to 0
    choice=0
 fi   
 fi
 fi
 done 

 LinuxBox# ./my_script.sh
 1. Shell
 2. Scripting
 3. ITTutorials
 Please enter a choice [1,2 or 3]? 3
 Your choice is: ITTutorials
 LinuxBox#

(Unless user enters integer value between 1-3, script will keep repeating itself.)

Until loop

Like the while loop, until loop is used for the same purpose of repetitive execution of code. Only difference is that condition that's controling loop is opposite. Loop will keep "looping" as long as the condition remains false, or in this case until it reaches value of defined counter:

Syntax of Until loop:

until [false]

do

execute commands

done

LinuxBox# vi my_script.sh
 #!/bin/bash
 # simple untill loop script, will keep looping until counter reaches value of 6
 COUNT=0
 # bash until loop
 until [ $COUNT -gt 5 ]; do
        echo Value of count is: $COUNT
        let COUNT=COUNT+1
 done

Select command

The select command is often used for creating an interactive terminal menus.

Syntax of Select statement:

select varName in list

do

command1

command2

......

commandN

done

Example of script with interactive terminal menu using select statement:

 LinuxBox# vi my_script.sh
 #!/bin/bash
 # select statement
 PS3='Choose one word: ' 
 # bash select
 select word in "linux" "shell" "scripting" "ittutorials" 
	do
   echo "The word you have selected is: $word"
 # Break, otherwise we'll have endless loop
	break  
 done
exit 0

( PS stands for prompt statement. It is shell's environment variable, and default prompt string used by select is "#?". It can be changed by re-defining PS3, so Select can display the string stored in PS3 when it is ready to read the user's selection like in the example.)

Case statement

Shell case statement is similar to switch statement in C and some other programing languages. It checks the Case condition, and controls the flow of the program. It can be used to test simple values like integers and characters.

Syntax of Case statement:

case expression in

do

pattern1) execute commands ;;

pattern2) execute commands ;;

....

patternN) execute commands ;;

esac

Example script with case statement:

LinuxBox# vi my_script.sh
 #!/bin/bash
 # simple Case example script
 echo "What is your preferred programming / scripting language"
 echo "1) bash"
 echo "2) php"
 echo "3) C++"
 echo "4) C"
 echo "5) Exit"
 read user_choice;

 case $user_choice in
     1) echo "You selected bash";;
     2) echo "You selected php";;
     3) echo "You selected C++";;
     4) echo "You selected C";;
     5) exit
 esac 

 LinuxBox# ./my_script.sh
 What is your preferred programming / scripting language
 1) bash
 2) php
 3) C++
 4) C
 5) I do not know !
 1
 You selected bash

Shell Functions

As in many other programming languages, usage of functions is also possible in shell scripting. Functions are used to group pieces of code in a more logical way, to enable calling that code anywhere inside the script and using it multiple times.

Declaring a function is done by using "function" keyword with the name of that function. For example, function my_func { my_code }. Inside "{" and "}" goes code that will be executed every time the function is called.

Calling a function is simple as writing function's name.

Example of script that uses functions:

LinuxBox# vi my_script.sh
 #!/bin/bash
 # script that uses functions
 function addition  {
    A=3
	B=5
	result=`expr $A + $B`
    exho "$A + $B = $result"
    exit
    }
 function hello {
    echo "Just saying Hello from function."
    }
  hello
  addition   

  LinuxBox# ./my_script.sh
  Just saying Hello from function.
  3 + 5 = 8

Arithmetic Comparison

Using relational operators we can easily compare numeric values. These operators would not work for string values unless their value is numeric. To see more about relational operators check Linux shell opertors tutorial.

Comparing 2 numbers using relational operators:

 LinuxBox# vi my_script.sh
 #!/bin/bash
 # comparing 2 integer numbers
 echo "Enter 1st umber:"
 read 1st
 echo "Enter 2nd umber:"
 read 2nd
 if [ $1st -eq $2nd ]; then
	echo "Both numbers are equal"
 elif [ $1st -gt $2nd ]; then
	echo "1st is greater then 2nd number"
 else 
	echo "2nd is greater then 1st number"
 fi 

 LinuxBox# ./my_script.sh
 Enter 1st umber:
 22
 Enter 2nd umber:
 5
 1st is greater then 2nd number

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Shell scripting - Logical structures loops and conditions

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