Sometimes you only want to run a command if a certain condition is true. For this, we have the if… then… else…fi construct in bash.
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Check arguments in bash script
We can use conditional programming to improve our previous example, arguments.sh, since it contained a little problem. It expects a name in $1 without checking if it actually gets one. Let’s fix this:
If that didn’t work, here’s a static version of the same code:
#!/bin/bash if test -z "$1" then echo "Usage: $0 <Your name>" else echo "Hello $1, from $0" fi
The test command in bash
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 so it really aids in conditional programming. The full list can be seen when you enter man test on the command line. 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. After all, in the old days, we didn’t have the Internet to google everything…
Here is one more example in which we compare two values to see if they are the same:
Here’s the static version in case that didn’t work:
#!/bin/bash
for i in {1..10}
do
if test $i -eq 3
then
echo "I found the 3!"
else
echo "Not looking for the $i"
fi
done
This loop runs for 10 iterations and checks if $i is equal to 3 each time. Can you predict the output?
Although the else-part is optional, you always need to end with a fi.
