Introduction to Tools and Techniques in Computer Science

Arguments

Franklin Bristow

Shell scripts that you write can accept arguments, just like programs you write in other programming languages. In both C and Java (and Python, technically), you can access arguments passed on the command line to your program as arrays of strings.

Shell scripts can access command line arguments using variables, but you can directly access arguments on the command line as numbered variables like $1.

Here’s a small program that will print out the values passed to it as arguments on the command line:

#!/usr/bin/env bash

echo "The first argument is $1"
echo "The second argument is $2"
echo "The third argument is $3"
echo "All arguments are $*"

Write this script and try running it with different arguments to see how the output changes. Don’t forget to use chmod to set execute permissions for your script!

A common use of arguments on the command line for shell scripts is to pass the name of a file or directory you want to operate on.

Let’s upgrade our la script a little bit.

Remember that ls can run with no arguments, and when run with no arguments it’s defaulting to printing out the contents of the directory .. But ls can accept arguments. Our la script doesn’t right now.

Change your la script to accept an argument and pass it to ls:

#!/usr/bin/env bash

ls -al "$1" # quotes in case of spaces!

Now run la again, but pass it an argument:

la .

Neat.

Why “quotes in case of spaces”? Try this:

  1. Remove the quotes around $1 in your script.
  2. Create a directory that has a space in its name (mkdir "space dir").
  3. Try running la "space dir".
  4. Put back the quotes around $1 in your script.

When the shell “expands” the variable $1, it’s replacing the value of that variable into the command literally. If the variable contains spaces, it will be replaced in the command spaces and all. In other words,

ls -al $1 # becomes:
ls -al space dir

If you remember way back a long time ago, we had to put quotes around names with spaces when using mkdir because mkdir would turn space dir into two directories. Similarly, ls is looking for two separate directories.

Including the quotes around $1 makes sure that even if the variable contains spaces, it’s going to be quoted when it’s passed to the command:

ls -al "$1" # becomes
ls -al "space dir"

While we’ve improved la slightly here, we’ve also broken it. Try running la by itself with no arguments.

…

Oops. Now we need to test for the special case of no arguments being passed. We’re going to need some more tools for that: conditional statements.