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Writing a Bash Script

Writing a Bash Script

By typing commands on the Linux command line, you can give the server instructions to get some simple tasks done. A shell script is a way to put together a series of instructions to make this easier. Shell scripts become even more powerful when you add logic like if and while to automatically control how they behave as circumstances change.

What’s Bash?

Bash is the name of a command line interpreter, a program that makes sense of the Linux commands you enter at the command prompt, or in your script. There are other interpreters as well, such us TC-Shell, Z Shell, Fish-Shell, and many more.

What’s in a Script?

A script is just a file. A basic script is made up of an introductory line, called a "shebang", that tells the server what to make of it, and one or more instructions to execute. Here’s an example:

#!/bin/bash
echo "Hi. I’m your new favorite bash script."

The first line has special meaning, which we'll discuss below. The second line is just a Linux command, one you could type out on the command line.

What’s a Comment?

Comments are text you add to your script that you intend bash to ignore. Comments start with a pound sign, and are useful for annotating your code so you and other users can understand it. To add a comment, type the # character, followed by any text that's helpful you. Bash will ignore the # and everything after it.

Note: the first line of the script is not a comment. This line is always first, always starts with #! and has special meaning to bash.

Here’s the script from before, commented:

#!/bin/bash # Designates the path to the bash program. Must start with '#!' (but isn't a comment).
echo "Hi. I’m your new favorite bash script." # 'echo' is a program that sends a string to the screen.

Executing a Script

You can open a text editor, paste that example code and save the file, and you’ve got a script. Scripts are conventionally named ending in '.sh,' so you might save that code as myscript.sh.

The script won’t execute until we do 2 things:

First, make it executable. (We’ll only have to do this once.) Linux relies extensively on file permissions. They determine a lot about how your server behaves. There's a lot to know about permissions, but for now we only need to know this: you can't run your script until you give yourself execute permissions. To do that, type:

chmod +x my script.sh

Second, run it. We execute the script from the command line just like any other command like ls or date. The script name is the command, and you need to precede it with a './' when you call it:

./myscript.sh # Outputs "Hi. I'm your new favorite bash script." (This part is a comment!)

Conditionals

Sometimes you want your script to do something only if something else is true. For example, print a message only if a value is below a certain limit. Here's an example of using if to do that:

#!/bin/bash

count=1 # Create a variable named count and set it to 1

if [[ $count -lt 11 ]]; then # This is an if block (or conditional). Test to see if $count is 10 or less. If it is, execute the instructions inside the block.
    echo "$count is 10 or less" # This will print, because count = 1.
fi # Every if ends with fi

Similarly, we can arrange the script so it executes an instruction only while something is true. We'll change the code so that the value of the count variable changes:

#!/bin/bash

count=1 # Create a variable named count and set it to 1

while [[ $count -lt 11 ]]; do # This is an if block (or conditional). Test to see if $count is 10 or less. If it is, execute the instructions inside the block.
    echo "$count is 10 or less" # This will print as long as count <= 10.
    count=$((count+1)) # Increment count
done # Every while ends with done

The output of this version of myscript.sh will look like this:

"1 is 10 or less"
"2 is 10 or less"
"3 is 10 or less"
"4 is 10 or less"
"5 is 10 or less"
"6 is 10 or less"
"7 is 10 or less"
"8 is 10 or less"
"9 is 10 or less"
"10 is 10 or less"

Real World Scripts

These examples aren't terribly useful, but the principles are. Using while, if, and any command you might otherwise type manually, you can create scripts that do valuable work.

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