Today we will look at how to restructure your bash scripts to use logical operators.

The best place to start is by saying that if you are writing an if conditional in your bash script, it can probably be replaced with more a more elegant solution using Bash’s built in logical operators. Obviously there are times and places in which if conditionals are unavoidable, however it is worth nothing that a lot of them can be avoided by understanding logical operators. Unlike other languages, a small caveat is that when a condition is met, a logical operator returns 0 status code for true, and 1 for false.

Logical Operators

Today we will be covering 4 logical operators. The operators we will cover are:

;
&&
||
&

;

The semi-colon is used in between commands and allows you to string multiple commands together, regardless of status code output.

cat ~/existingfile.txt ; echo "existingfile opened"

Assuming ~/.existingfile.txt does not exist, will print:

cat: /home/default/existingfile.txt: No such file or directory
existingfile opened

However if it does exist:

abcdefghijklmnopqrstuvyxwz
existingfile opened

&&

and-and, unlike ;, will only perform the second command if the first command returns a 0 (True) status code.

cat ~/existingfile.txt && echo "existingfile opened"

Assuming existingfile.txt exists, outputs:

abcdefghijklmnopqrstuvyxwz
existingfile opened

If however this file does not exist, the first command returns a non 0 status codes, therefore the output is:

cat: /home/default/existingfile.txt: No such file or directory

||

As you would expect, pipe-pipe does the inverse of &&. If the expression on the left evaluates to a non 0 status code, perform the action on the right.

cat ~/existingfile.txt || echo "existingfile does not exist"

Assuming ~/.existingfile.txt does not exist, will print:

cat: /home/default/existingfile.txt: No such file or directory
existingfile does not exist

and if it does exist:

abcdefghijklmnopqrstuvyxwz

&

This operator bares no resemblemces to && even though you would assume it does. & still takes 2 expressions, however it executes the right hand side without waiting for the left hand side to complete. For example:

sleep 5 & echo "Finished first"

outputs:

default:[~/]:$ sleep 5 & echo "Finished first"
[1] 29766
Finished first

“Finished first” gets printed immediately and returns its process id.

Practical Example

Lets refactor this basic if command which is a deploy script to an environment:

if [ "$env" != "development" ] || \
    [ "$env" != "staging" ] || \
    [ "$env" != "production" ] ; then
    deploy $env
elif
    error_message

On the face of it, this example seems to be okay. However, taking what we have learnt (plus a few extra tricks which I will discuss briefly), we can use logical operator to make it a simple, elegant one liner.

[[ ! $env =~ ^(development|staging|production)$ ]] && deploy $env || error_message

Firstly, we can actually remove the if declaration as it is actually implicit with the [] declaration. Secondly, we added the if syntax improvement [[]], which has some nice added features on its predecessor, []. Namely, you can declare variables with the use of quotations, which we have used here. It also allows us to use the =~ operator, which allows us to use regular expressions (^ denotes the start and $ the end). Instead of searching through if something doesn’t equal this or something doesnt equal that, we can declare the ! to start with to negate the following expression.

Now, if our new expression gives us a 0 status code, we can deploy to the environment. If the preceding expression gives us a 1 status code, then our deploy $env will have a 0 status code, meaning the program will execute the error_message function.