bash script
Linux

Bash Script Step By Step, You will love it

Today we are going to talk about bash scripting or shell scripting and how to write your first bash script. Actually, they are called shell scripts in general, but we are going to call them bash scripts because we are going to use bash among the other Linux shells.

There are zsh, tcsh, ksh and other shells.

So far you’ve seen how to use the bash shell and how to use Linux commands.

The idea of a bash script is the ability to enter multiple Commands and deal with the results from each command. The shell allows you to run multiple commands in a single step.

If you want to run two commands together or more, you can type them on the same line, separated by a semicolon.

pwd ; whoami

Actually, what you have typed is a bash script!!

This simple script uses just two bash shell commands.

The pwd command runs first, displaying the current working directory followed by the output of the whoami command, showing who is currently logged in user.

Using this technique, you can string together as many commands as you wish, but with a limit. You can determine your max args using this command.

 getconf ARG_MAX

Well, that’s fine, but it has a problem, you must enter the command at the command prompt every time you want to run it, what about putting the commands into a file, and when we need to run these commands we run that file only. This is called a bash script.

Now, create an empty file using the touch command. At the beginning of any bash script we write, we should define which shell we will use because there are many shells on Linux, bash shell is one of them.

Bash Script Shebang

The first line you type when writing a bash script is the (#!) followed by the shell you will use.

The pound sign followed by the exclamation mark is called shebang.

#!/bin/bash

In bash scripting, the pound sign (#) is used to comment a line which is not processed by the shell. However, the first line of any shell script file is different. This line specifies what shell we will use, which is bash shell in our case.

The shell commands are entered one per line, and you can write a comment by adding the pound sign at the beginning of the file like this:

You can use the semicolon and put multiple commands on the same line if you want to, but it is preferable to write commands on separate lines, this will make it simpler to read later.

Set Script Permission

After writing your bash script, save the file.

Now, all you need to do is to set that file to be executable in order to be able to run it, otherwise, it will give you permissions denied. You can review how to set permissions using chmod command.

bash script permission

chmod +x ./myscript

Then try run it by just typing it in the shell:

./myscript

And Yes, it is executed.

bash script first run

Print Messages

As we know from other posts, printing text is done by echo command.

Edit our file and type this:

Look at the output:

bash script echo command

Perfect! Now we can run commands and display text using echo command.

If you don’t know echo command or how to edit a file I recommend you to view previous articles about basic Linux commands

Using Variables

Variables allow you to store information in the bash script for use with other commands in the script.

There are two types of variables you can use in your bash script:

  • Environment variables
  • User variables

Environment Variables

Sometimes you need to interact with system variables in your shell commands to process information. You can do this by using environment variables.

Notice that we put the $HOME system variable in the double quotation marks in the first string, and the shell script still knows it.

bash script global variables

What if we want to print the dollar sign itself?

echo "I have $1 in my pocket"

The script sees a dollar sign within quotes; it assumes you’re referencing a variable. The script attempts to display the variable $1 which is not defined so how to overcome that?

By using the escape character which is the backslash \ before the dollar sign like this:

echo "I have \$1 in my pocket"

Now the bash script will print the dollar sign as it is.

bash script escape dollar sign

User variables

In addition to the environment variables, a bash script allows you to set and use your own variables in the script.

Variables defined in the shell script maintain their values till bash script execution finished.

Like system variables, user variables can be referenced using the dollar sign:

 

bash script user variables

Command substitution

One of the best features of bash scripts is the ability to extract information from the output of a command and assign it to a variable so you can use that value anywhere in your script.

There are two ways to do that:

  • The backtick character `
  • The $() format

Make sure when you type backtick character, it is not the single quotation mark.

You must surround the command with two backtick characters like this:

mydir=`pwd`

Or the other way

mydir=$(pwd)

So the script could be like this:

The output of the command will be stored in that variable called mydir.

bash script command substitution

Math calculation

You can perform basic math calculations using $(( 2 + 2 )) format:

Just that easy.

bash script math

if-then statement

Bash script requires some sort of logic flow control between the commands in the script. Like if the value is greater than 5 do this else do that. You can imagine any logic you want.

The most basic structure of if-then statement is like this:

if command

then

commands

fi

and here is an example:

If the command completes without problems, the echo statement should display the text.

Let’s dig deeper and use other commands we know.

Maybe searching for a specific user in the user’s file /etc/passwd and if a record exists, it prints a message saying that the user is present.

bash script if-else

We use the grep command to search for the user in /etc/passwd file. You can check our tutorial about the grep command.

If the user exists, the bash script will print the message.

What if the user doesn’t exist? The script will exit the execution without telling us that the user doesn’t exist. OK, let’s improve the script more.

if-then-else Statement

The if-then-else statement looks like this:

if command

then

commands

else

commands

fi

If the first command runs and returns zero; which means success, it will not hit the commands after the else statement, otherwise, if the if statement returns non-zero; which means the statement condition fails, in this case, the shell will hit the commands after else statement.

bash script if-else

We are doing good till now, keep moving.

Now, what if we need more else statements.

Well, that is easy, we can achieve that by nesting if statements like this:

if command1

then

commands

elif command2

then

commands

fi

If the first command return zero; means success, it will execute the commands after it, else if the second command return zero, it will execute the commands after it, else if none of these return zero, it will execute the last commands only.

You can imagine any scenario here, maybe if the user doesn’t exist, create a user using the useradd command or do anything else.

Numeric Comparisons

You can perform a numeric comparison between two numeric values using numeric comparison checks like this:

n1 -eq n2 Checks if n1 is equal to n2

n1 -ge n2 Checks if n1 is greater than or equal to n2

n1 -gt n2 Checks if n1 is greater than n2

n1 -le n2 Checks if n1 is less than or equal to n2

n1 -lt n2 Checks if n1 is less than n2

n1 -ne n2 Checks if n1 is not equal to n2

As an example, we will try one of them and the rest is the same.

Note that the comparison statement is in square brackets as shown.

bash script numeric compare

The val1 is greater than 5 so it will run the first statement and prints the first echo.

String Comparisons

Performing comparisons on strings are so easy. The comparison functions used to evaluate two string values are:

str1 = str2 Checks if str1 is the same as string str2

str1 != str2 Checks if str1 is not the same as str2

str1 < str2 Checks if str1 is less than str2

str1 > str2 Checks if str1 is greater than str2

-n str1 Checks if str1 has a length greater than zero

-z str1 Checks if str1 has a length of zero

We can apply string comparison on our example

bash script string compare

One tricky note about the greater than and less than for string comparisons, they MUST be escaped with the backslash because if you use the greater-than symbol only, it shows wrong results.

The script interpreted the greater-than symbol as an output redirection. So you should do it like that:

bash script string greater than

It runs but it gives this warning:

./myscript: line 5: [: too many arguments

To fix it, wrap the $vals with a double quotation, forcing it to stay as one string like this:

bash script string fix

Last tricky note about greater than and less than for string comparisons is when working with uppercase and lowercase letters, the sort command handles uppercase letters opposite to the way the test conditions doing.

bash script character case

sort myfile

likegeeks

Likegeeks

bash script sort order

Capitalized letters are treated as less than lowercase letters in test comparisons. However, the sort command does exactly the opposite.

String test comparisons use standard ASCII ordering, which means using each character’s ASCII numeric value to determine the sort order.

Unlike the sort command which uses the ordering defined in the system locale language settings.

File Comparisons

There are many file comparisons that you can do in the bash script:

-d file Checks if the file exists and is a directory

-e file Checks if the file exists

-f file Checks if the file exists and is a file

-r file Checks if the file exists and is readable

-s file Checks if the file exists and is not empty

-w file Checks if the file exists and is writable

-x file Checks if the file exists and is executable

file1 -nt file2 Checks if file1 is newer than file2

file1 -ot file2 Checks if file1 is older than file2

-O file Checks if file exists and is owned by the current user

-G file Checks if file exists and the default group is the same as the current user

As they imply, you will never forget them.

Let’s pick one of them and take it as an example:

bash script file checking

We are not going to type every one of them as an example. You just type the comparison between the square brackets as it is and complete you script normally.

There are some other advanced if-then features but let’s make it in another post.

That’s for now. I hope you enjoy it and keep practicing more and more.

Thank you.