Break, Continue and More Commands In Shell Script

Break

-> Suppose we want that our loop should stop running when certain conditions meet then we use Break.

Example - Suppose we want to find a particular number from a list of numbers and if that number is found then we should come out of the loop.

#!/bin/bash

no=6

for i in 1 2 3 4 5 6 7
do
        if [[ $no -eq $i ]]
        then
            echo " $no No is Present"
            break
        fi
done

Output -

Continue

Suppose a situation comes where if a condition is met then we do not want to perform the operation and want to move forward then we can use continue.

Example - Suppose we have a list of numbers from 1 to 10. We want that only even number should be printed then we can use continue for that -

#!/bin/bash

for i in 1 2 3 4 5 6 7 8 9 10
do
        if [[ $i%2 -ne 0 ]]
        then
                continue
        else
                echo "Number is $i"
        fi
done

Output -

Some more commands

exit -> It is used to stop the script at a point

Ex -

#!/bin/bash
if [[ $# -eq 0 ]]
then
        echo "Please Enter Some arguments"
        exit 1
fi
echo "First Argument is $1"

echo "Second Argument is $2"

-> Here it is dependent on the Arguments, if we don't provide then our script will not run.

Output -

-> To see the exit status - $?

-> When our script fails to execute then the status code we get is a non-zero value-

-> When our script sun successfully then we get zero value

basename - Strip Dirctory info and only give filename

dirname - Strip filename info and only give Directory path

realpath - Gives us a full path of a file. It will give full path of location were we are.

if [[ -d folder_name ]] -> Check if directory exist

if [[ ! -d folder_name ]] -> Check if directory not exist

if [[ -f file_name ]] -> Check if file exist

#!/bin/bash

FILEPATH="/home/ubuntu/dir1/dir2/dir3/file1.txt"

if [[ -f $FILEPATH ]]
then
        echo "File exists"
else
        echo "File not exists"
fi

if [[ ! -f file_name ]] -\> Check if file not exist

Bash Variable

RANDOM

- It generates integer between 0 and 32767

UID

- It give the User ID of the user Logged In.

For root user, id will be zero

-> We can write a script to check if the User is logged in as a root or not-

#!/bin/bash

if [[ $UID -eq 0 ]]
then
        echo "User is logged in as a root"
else
        echo "User is not a root user"
fi

Output -

Redirection In Shell Script

-> We can redirect the output of a commands in other file

Threre are two ways in which we can redirect

1) Using ">" - If we use this then when we enter a new content in a file then earlier content will be deleted.

-> We have created a file which is all_file.txt and storing the output of ls command.

-> We can see that we have all the file list in all_file.txt file

-> Now when we storing the output of echo command in this file then we can see that our previous data got lost.

2) Using ">>" - If we want to persist all the content in this file then we can use this.

/dev/null

Suppose we do not want to print the output of a command on a terminal and do not want to store in a file then we can use &> /dev/null.

Example - Suppose if we do not use /dev/null then

-> When we use /dev/null

Logger

-> if we want to maintain the logging for our script then we can use logger in our script.

-> We can find logs under /var/logs/messages

Example - logger "Hello Bhaskar"

set -x

-> To enable the debugging in the script, we can use set -x in the start of the script.

set -e

-> If the script fails then it will exit and stop running the script

Run the Script In the Background

-> When we run the script then it captures the terminal and that time we can not perform other task.

-> To avoid this situation we can run this script in background,

-> To do this we have to use nohup command.

Example - nohup ./script_name &

Note - First chnage the permission of the file

-> Here it will create a nohup.out file and store the output of the script.

#!/bin/bash

for i in 1 2 3 4 5 6
do
        echo "Number is $i"
        sleep 3s
done

Output -