Shell Scripting

In this section will look at how to create shell scripts, in order to run them we will make them executable - just like commands.

Shell scripts are files created using a text editor such as Nano, the files are saved with a .sh extension to indicate that they are shell scripts.

The steps below step through how to create a simple shell script.

Step 1: The Interpreter Directive

Open a new file in Nano, enter the commands below and save as hello_world.sh

#!/bin/bash
# This is a very simple hello world script.
echo "Hello, world!"

• #! - is used to indicate which program should be used to interpret the script
  • In this case it’s bash, it could also be a language like python, perl, ruby etc.
  • This must be the first line in the script
• # - a comment (ignored by /bin/bash)

Step 2: Making it executable

We can use chmod to change permissions on the script and make it executable:

[user@host ~]$ chmod u+x hello_world.sh
[user@host ~]$ ls -l hello-world.sh
-rwxr--r-- 1 user ccaas0 30 Mar 31 17:10 hello_world.sh

Step 3: Location

To run the script:

[user@host ~]$ ./hello_world.sh
hello world!

What’s with the “./” ?

Remember the PATH! - we are instructing bash to run the script in the current location.

Changing the PATH

If you want to be able to make your script work like a command, you need the directory it is in to be in your PATH

[user@host ~]$ mkdir ~/scripts
[user@host ~]$ PATH=$PATH:$HOME/scripts
[user@host ~]$ export PATH

• Create a directory in your home directory specifically for your scripts
• Add this directory to the PATH environmental variable
• Make sure the scripts you want to use are executable

Command line arguments

You can control your script’s behaviour with arguments you pass to it when you run it.

An example might be:

[user@host ~]$ ./script.sh var1 var2

Within the script: $1 contains “var1” $2 contains “var2”

The script looks like this:

#!/bin/bash

echo The first argument is $1
echo The second argument is $2
echo And together they make ${1}${2}

And here it is in use:

[user@host ~]$ ./var-script green house
The first argument is green
The second argument is house
And together they make greenhouse

Next activity: Exercises - File permissions.