bash

An umount command is used to unmount a device/partition by specifying the path to the directory where it has been mounted.

Sometimes, when you run the umount command you may receive the “target is busy” or “device is busy” errors indicating that there is some process that is using the mounted filesystem or the remote file server is not responding for some reason.

Luckily, it is possible to run the umount command forcefully and this short note shows how to do this safely. (more…)

Sometimes you might need to run some command from the Linux command line and repeat it several times.

There is a simple way to write a command once and have it executed N times using Bash loop FOR.

In this short note you’ll find how to execute some command a number of times in a row. (more…)

If you often create Bash scripts, you may sometimes need to get the length of a string or find out the length of a variable that stores some string.

This short note shows how to find out the length of a string from the Linux command line and how to count the length of the each line in a file.

You will also see how to get the length of a string if it is stored in a variable in Bash and then assign that calculated number of characters to another variable. (more…)

Very often in bash scrips you need to ask for user input that requires a Yes or No answer.

For example, you may want to put a quick “Are you sure?” prompt for confirmation before execution of some potentially dangerous part of a bash script.

In this article you’ll find three easiest and fastest ways to prompt for “Yes/No” confirmation in bash script.

Prompt To Continue In Bash

The best way to prompt for a confirmation to continue in a bash script is to use the read command (source):

read -p "Are you sure? " -n 1 -r
echo    # (optional) move to a new line
if [[ ! $REPLY =~ ^[Yy]$ ]]
then
    exit 1
fi

Simple “Yes/No” Dialog In Bash

One of the most widely used method to ask user for a confirmation in a bash script is to combine read + case commands (source):

while true; do
    read -p "Do you wish to install this program?" yn
    case $yn in
        [Yy]* ) make install; break;;
        [Nn]* ) exit;;
        * ) echo "Please answer yes or no.";;
    esac
done

Select From “Yes/No” Menu In Bash

Another easy method is to use select command (source):

echo "Do you wish to install this program?"
select yn in "Yes" "No"; do
  case $yn in
    Yes ) make install;;
    No ) exit;;
  esac
done

The following article describes the basic syntax and includes a simple example of the BASH CASE statement usage.

The CASE statement is the simplest form of the IF-THEN-ELSE statement in BASH.

You may use the CASE statement if you need the IF-THEN-ELSE statement with many ELIF elements.

With the BASH CASE statement you take some value once and then test it multiple times.

Basic Syntax of the CASE Statement

case $variable in
     pattern-1)      
          commands
          ;;
     pattern-2)      
          commands
          ;;
     pattern-3|pattern-4|pattern-5)
          commands
          ;; 
     pattern-N)
          commands
          ;;
     *)
          commands
          ;;
esac

Example of a BASH Script with the CASE Statement

#!/bin/bash
printf 'Which Linux distribution do you know? '
read DISTR

case $DISTR in
     ubuntu)
          echo "I know it! It is an operating system based on Debian."
          ;;
     centos|rhel)
          echo "Hey! It is my favorite Server OS!"
          ;;
     windows)
          echo "Very funny..."
          ;; 
     *)
          echo "Hmm, seems i've never used it."
          ;;
esac

Run the script as follows:

$ ./testcase.sh
Which Linux distribution do you know? centos
Hey! It is my favorite Server OS!
$ ./testcase.sh
Which Linux distribution do you know? rhel
Hey! It is my favorite Server OS!
$ ./testcase.sh
Which Linux distribution do you know? ubuntu
I know it too! It is an operating system based on Debian.
$ ./testcase.sh
Which Linux distribution do you know? pfff
Hmm, seems i've never used it.

Use the following test in your shell scripts to:

1. verify the number of input values

2. display an error message if the number of input argument is not correct

3. exit a shell script with the error status

[ $# -eq 0 ] && { echo "Usage: $0 argument"; exit 1; }

Bash Script Example

The following script is using dig command to find the name server of a domain name. The domain name should be provided as an argument.

#!/bin/bash
domain=$1
[ $# -eq 0 ] && { echo "Usage: $0 domain_name"; exit 1; }
dig NS $domain @8.8.8.8 +short

Sample output if no arguments are specified :

$ ./find_ns.sh
Usage: ./find_ns.sh domain_name

Sample output if an argument is passed:

$ ./find_ns.sh shellhacks.com
ns2.ukraine.com.ua.
ns3.ukraine.com.ua.
ns1.ukraine.com.ua.