∙ Shell scripts

Files containing commands - a step towards programming

Contents

Scripts

So far, we have been running things directly on the command line, “interactively”. However, you might want to keep your commands in a file, and be able to run, share, modify, or use files as templates. This is the gateway to coding - and is great practice for developing your reproducible work, for communicating your methods with others, and for archiving your work.

A “script” is a file of instructions that can be directly read and run by a program. For example, a file of Python code is a script, since it does not need to be compiled into binary - this is done by the Python interpreter, working as it goes along. In our case, we are going to make a script, but the language will be shell (all of the commands we have been using so far are shell commands).

Let’s create a very simple script - open nano, creating a file called “my-first-script.sh” (or anything you like!) and include two lines

ls -l *part*
echo "That is all of the files with \"part\" in that I could find here."

Info

If you didn’t look up echo earlier, use man to work out what it does.
Think of echo as print, used commonly in programming languages.

Save the file and exit nano. Now, we are going to run this file, in the folder some_plays. To do this, we are going to ask the program bash (“Bourne Again shell”, a widely-used shell program) to run it, with

bash my-first-script.sh

Exercise

Let’s use all of the skills we have learned today to build a script to complete a task. Be in the folder some_plays. Use nano to start a new, empty file. Create a script that will:

Exercise

Print the final line of Macbeth
Count how many time the word “merry” occurs in all plays starting with the letter “m”
Count how many times Hamlet speaks in Hamlet
Print the line of who edited each play
Use head and tail to print just the 500th line of All’s Well That Ends Well
Make a new file, in the folder containing some_plays, of the total count of the lines containing the word “love” in all of the plays (combined - ie a single number).

Tip

Test each of your commands individually on the command line, before building them into your script.

If you can complete this task - congratulations, you now know all of the important ways of navigating the file system, of searching files, and the basics of shell scripting.

Learn more

When learning to use computer clusters and high-performance computers, we often run jobs using “submission scripts”. These are just the same as a shell script - they are not magic, merely running commands one after another, just like you could on the command line itself.

You can learn these skills in our course Introduction to HPC, do an internet search for our current courses.