Bash Conditional Programming

Sometimes you only want to run a command if a certain condition is true. For this, we have the if… then… else…fi construct.

Our previous arguments.sh example contained a little problem. It expects a name in $1 without checking if it actually gets one. Let’s fix this:

#!/bin/bash
if test -z "$1"
then
  echo "Usage: $0 <Your name>"
else
  echo "Hello $1, from $0"
fi

With test -z we can check if a variable’s length is zero. If that is the case, we print some friendly usage instructions:

$ ./arguments.sh
Usage: ./arguments.sh <Your name>

The test command can test for many things. The full list can be seen when you enter

$ man test

That’s right, you don’t need Google for everything! Using man-pages is part of being a command-line ninja! You’ll find that there’s a man page for pretty much everything you can do in your terminal.

Here is one more example in which we compare two values to see if they are the same:

#!/bin/bash
for i in {1..10}
do
  if test $i -eq 3 
  then
    echo "I found the 3!"
  fi
done

This loop runs for 10 iterations and checks if $i is equal to 3 each time. Can you predict the output?

As you can see, the else-part is optional, however, you always need to end with a fi.

About the author

Erik van Baaren