Shell Scripting

Shell is like a container, that accepts commands from the user/other programs and sends them to the kernel after parsing and processing.

Shell

In simple words, Shell is an application that takes input from the keyboard, and passes that to the kernel for processing.

If you want to check available shells on your Linux OS, check the “/etc/shells” file.

$ cat /etc/shells

# /etc/shells: valid login shells
/bin/sh
/bin/bash
/usr/bin/bash
/bin/rbash
/usr/bin/rbash
/usr/bin/sh
/bin/dash
/usr/bin/dash
/usr/bin/tmux
/usr/bin/screen

Here we can see a list of all available shells.

To check which shell you are using right now, use the following command-

# Check shell that is currently running
$ echo $0

-bash

# Check  the default shell assigned to this user
$ echo $SHELL

/bin/bash

# Check the process the currently running shell
$ ps -p $$

  PID TTY          TIME CMD
55782 pts/10   00:00:00 bash

You can also check the “/etc/passwd” file. In the file, you can check which shell is defined by default for your user name.

$ cat /etc/passwd

root:x:0:0:root:/root:/bin/bash
bigboxcode:x:1000:1000:,,,:/home/bigboxcode:/bin/bash
redis:x:108:118::/var/lib/redis:/usr/sbin/nologin

If you want to change the default shell by making changes in this file.

Bash

The original Linux shell was written(developed) by Steve Bourne.

Bash is an acronym of- bourne-again shell

Bash is an enhanced version of shell(sh).

Kernel

Kernel is the interface between hardware and software. Kernel stays on top of the hardware as a layer, takes every command provided by Shell, and executes that on the hardware.

Shell Script Permission

Script files should have proper permission for execution. Give them the script execution permission by using the following command-

$ chmod a+x name_of_the_script

Denote Shell

At the beginning of the script, we have to denote the shell, for which this script is written for.

For all our scripts we will write for “sh”. So we will write the following line at the beging of every script-

#!/bin/sh

Hello World

Let’s start with a simple example of print “hello world”. We can use “echo” to output the “Hello world” on the script.

#!/bin/sh

echo Hello World

Save the file as “script1.sh” and then run it, you will get output as below-

$ ./script1.sh

Hello World

echo

Use the echo command to print something on the screen. We can also use “echo” without any param/string to print empty line-

#!/bin/sh

echo
echo Hello World
echo

Run the script and you will see the output like below-

$ ./script1.sh

Hello World

Variable

Declaring a variable is very simple. Just write the name of the variable, then the equal sign and then the value.

name=bigboxcode
url="https://bigboxcode.com"
description="A long description of the site"

show_list="ls -la"

To use the variable use a dollar sign($) before the variable name.

Like, if we want to print the values then we can do it like below-

name=bigboxcode
url="https://bigboxcode.com"
description="A long description of the site"

show_list="ls -la"

echo Name: $name
echo $url
echo $description
echo $show_list

To use the variable value as a command, like we have “show_list” here, just add the dollar sign($) before the variable name and put it in a new line-

show_list="ls -la"

$show_list

Print Command Output

If we want to print the output of a command then we have to apply a specific format for that.

First, let’s see what happens if we just put the command directly in the “echo” command-

hn1=hostname

echo Host name: $hn1

echo Host name: hostname

Output:

Host name: hostname
Host name: hostname

To execute the command “hostname” we have to wrap it in backtick(`).

hn2=`hostname`

echo Host name: $hn2

echo Host name: `hostname`

Output:

Host name: bigboxcode
Host name: bigboxcode

We can do the same thing with other commands also-

echo List of files: `ls`
echo List of files: `ls -l`

Output:

List of files: script1.sh script2.sh script3.sh
List of files: 
total 12 

-rwxr-xr-x 1 bigboxcode bigboxcode 38 Jul 22 12:38 script1.sh 
-rwxr-xr-x 1 bigboxcode bigboxcode 193 Jul 22 13:41 script2.sh 
-rwxr-xr-x 1 bigboxcode bigboxcode 192 Jul 22 14:03 script3.sh

Read Input

Use the command “read” to get input from the user and save it in a variable. Later we can use that variable as a normal variable-

echo Enter your name here:

read name

echo Enter age:

read age

echo Hello, $name
echo Your age is: $age

Output:

Enter your name here:
Big Box Code
Enter age:
18

Hello, Big Box Code
Your age is: 18

if-else Condition

We can write an if-else condition in the script.

Compare Values

If you want to check if something is equal to some value use “-eq”.

mark=100

if [ $mark -eq 100 ] 
    then 
        echo In Then
        echo Mark is 100
    else
        echo In Else
        echo Mark is not 100
fi

Output:

In Then
Mark is 100

We can also take mark as input from the user and compare it-

echo Enter Mark:

read mark

if [ $mark -eq 100 ] 
then 
    echo In Then
    echo Mark is 100
else
    echo In Else
    echo Mark is not 100
fi

Output:

Enter Mark:
30
In Else
Mark is not 100

Here are some more comparison examples. Here we are checking the quality of the mark. We are checking if the mark is greater than(-gt) and greater than or equal(-ge).

echo Enter mark:

read mark

if [ $mark -gt 100 ]; then
    echo Invlaid number
elif [ $mark -ge 90 ]; then 
    echo Excellent
elif [ $mark -ge 80 ]; then
    echo Good
elif [ $mark -ge 60 ]; then  
    echo OK
else
    echo Not Good
fi

Output:

Run the script and enter some values. Here we have entered 80.

Enter mark:
80
Good

Check File Existence

Let’s see how we can check if a file exists. Write a script to check if the file “crash.log” exists or not-

if [ -e ./crash.log ]
then
    echo file exists
else
    echo file does not exist
fi

Output:

file does not exist

Now create file named “crash.log”.

touch crash.log

And run the script again-

Output:

file exists

case Statement

Start a case statement with “case” and end with “esac”. To ensure default option(when none of the options match) use asterisk(*). Use pipe(|) to use multiple options as “or”.

echo Enter option:

read option

case $option in
    a) echo You pressed a;;
    b|c|d) echo you pressed one of b or c or d;;
    *) echo none of the option matches;; 
esac

Output:

Enter option:
c
you pressed one of b or c or d

Enter option:
sdfd
none of the option matches

for Loop

for superhero in bat super spider iron snow
do
    echo Name of a superhero: ${superhero}man
done

Specify the end of the for loop with “done”.

Output:

Name of a superhero: batman
Name of a superhero: superman
Name of a superhero: spiderman
Name of a superhero: ironman
Name of a superhero: snowman

while Loop

count=0

while [ $count -lt 10 ]
do
    echo $count

    sleep 1

count=`expr $count + 1`
done

Output:

0
1
2
3
4
5
6
7
8
9

Exist Status

0 = OK or successful

1 = Minor problem

2 = Serious trouble

3-255 = Everything else

#!/bin/sh

ls -l ./crash.log

# Check status code of the last command execution by $?
if [ $? -eq 0 ]; then
    echo File found in the system
else
    echo File not found
fi

Output:

-rw-r--r-- 1 bigboxcode bigboxcode 0 Jul 22 16:26 ./crash.log
File found in the system