How to Use Bash If Statements (With 4 Examples)

All non-trivial Bash scripts need to make decisions. The Bash if statement lets your Linux script ask questions and, depending on the answer, run different sections of code. Here’s how they work.

What Is Conditional Execution?

In all but the most trivial of Bash scripts, there’s usually a need for the flow of execution to take a different path through the script, according to the outcome of a decision. This is called conditional execution.

One way to decide which branch of execution to take is to use an if statement. You might hear if statements called if then statements, or if then else statements. They’re different names for the same thing.

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The if statement says that if something is true, then do this. But if the something is false, do that instead. The “something” can be many things, such as the value of a variable, the presence of a file, or whether two strings match.

Conditional execution is vital for any meaningful script. Without it, you’re very limited in what you can get your script to do. Unless it can make meaningful decisions you won’t be able to take real-world problems and produce workable solutions.

The if statement is probably the most frequently used means of obtaining conditional execution. Here’s how to use it in Bash scripting.

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A Simple If Statement Example

This is the canonical form of the simplest if statement:

if [ this-condition-is-true ]
then
  execute-these-statements
fi

If the condition within the text resolves to true, the lines of script in the then clause are executed. If you’re looking through scripts written by others, you may see the if statement written like this:

if [ this-condition-is-true ]; then
  execute-these-statements
fi

Some points to note:

• The if statement is concluded by writing fi.
• There must be a space after the first bracket ” [ ” and before the second bracket ” ] ” of the conditional test.
• If you’re going to put the then keyword on the same line as the conditional test, make sure you use a semi-colon ” ; ” after the test.

We can add an optional else clause to have some code executed if the condition test proves to be false. The else clause doesn’t need a then keyword.

if [ this-condition-is-true ]
then
  execute-these-statements
else
  execute-these-statements-instead
fi

This script shows a simple example of an if statement that uses an else clause. The conditional test checks whether the customer’s age is greater or equal to 21. If it is, the customer can enter the premises, and the then clause is executed. If they’re not old enough, the else clause is executed, and they’re not allowed in.

#!/bin/bash

customer_age=25

if [ $customer_age -ge 21 ]
then
  echo "Come on in."
else
  echo "You can't come in."
fi

Copy the script from above into an editor, save it as a file called “if-age.sh”, and use the chmod command to make it executable. You’ll need to do that with each of the scripts we discuss.

chmod +x if-age.sh

Let’s run our script.

./if-age.sh

Now we’ll edit the file and use an age less than 21.

customer_age=18

Make that change to your script, and save your changes. If we run it now the condition returns false, and the else clause is executed.

./if-age.sh

The elif Clause

The elif clause adds additional conditional tests. You can have as many elif clauses as you like. They’re evaluated in turn until one of them is found to be true. If none of the elif conditional tests prove to be true, the else clause, if present, is executed.

This script asks for a number then tells you if it is odd or even. Zero is an even number, so we don’t need to test anything.

All other numbers are tested by finding the modulo of a division by two. In our case, the modulo is the fractional part of the result of a division by two. If there is no fractional part, the number is divisible by two, exactly. Therefore it is an even number.

#!/bin/bash

echo -n "Enter a number: "

read number

if [ $number -eq 0 ]
then
  echo "You entered zero. Zero is an even number."
elif [ $(($number % 2)) -eq 0 ]
then
  echo "You entered $number. It is an even number."
else
  echo "You entered $number. It is an odd number."
fi

To run this script, copy it to an editor and save it as “if-even.sh”, then use chmod to make it executable.

Let’s run the script a few times and check its output.

./if-even.sh

That’s all working fine.

Different Forms of Conditional Test

The brackets ” [] ” we’ve used for our conditional tests are a shorthand way of calling the test program. Because of that, all the comparisons and tests that test supports are available to your if statement.

This is just a few of them:

• ! expression: True if the expression is false.
• -n string: True if the length of the string is greater than zero.
• -z string: True if the length of the string is zero. That is, it’s an empty string.
• string1 = string2: True if string1 is the same as string2.
• string1 != string2: True if string1 is not the same as string2.
• integer1 -eq integer2: True if integer1 is numerically equal to integer2
• integer1 -qt integer2: True if integer1 is numerically greater than integer2
• integer1 -lt integer2: True if integer1 is numerically less than integer2
• -d directory: True if the directory exists.
• -e file: True if the file exists.
• -s file: True if the file exists with a size of more than zero.
• -r file: True if the file exists and the read permission is set.
• -w file: True if the file exists and the write permission is set.
• -x file: True if the file exists and the execute permission is set.

In the table, “file” and “directory” can include directory paths, either relative or absolute.

The equals sign “=” and the equality test -eq are not the same. The equals sign performs a character by character text comparison. The equality test performs a numerical comparison.

We can see this by using the test program on the command line.

test "this string" = "this string"

test "this string" = "that string"

test 1 = 001

test 1 -eq 001

In each case, we use the echo command to print the return code of the last command. Zero means true, one means false.

Using the equals sign ” = ” gives us a false response comparing 1 to 001. That’s correct, because they’re two different strings of characters. Numerically they’re the same value—one—so the -eq operator returns a true response.

If you want to use wildcard matching in your conditional test, use the double bracket ” [[ ]] ” syntax.

#!/bin/bash

if [[ $USER == *ve ]]
then
  echo "Hello $USER"
else
  echo "$USER does not end in 've'"
fi

This script checks the current user’s account name. If it ends in “ve“, it prints the user name. If it doesn’t end in ” ve “, the script tells you so, and ends.

./if-wild.sh

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Nested If Statements

You can put an if statement inside another if statement.

This is perfectly acceptable, but nesting if statements makes for code that is less easy to read, and more difficult to maintain. If you find yourself nesting more than two or three levels of if statements, you probably need to reorganize the logic of your script.

Here’s a script that gets the day as a number, from one to seven. One is Monday, seven is Sunday.

It tells us a shop’s opening hours. If it is a weekday or Saturday, it reports the shop is open. If it is a Sunday, it reports that the shop is closed.

If the shop is open, the nested if statement makes a second test. If the day is Wednesday, it tells us it is open in the morning only.

#!/bin/bash

# get the day as a number 1..7
day=$(date +"%u")

if [ $day -le 6 ]
then
  ## the shop is open
  if [ $day -eq 3 ]
  then
    # Wednesday is half-day
    echo "On Wednesdays we open in the morning only."
  else
    # regular week days and Saturday
   echo "We're open all day."
  fi
else
  # not open on Sundays
  echo "It's Sunday, we're closed."
fi

Copy this script into an editor, save it as a file called “if-shop.sh”, and make it executable using the chmod command.

We ran the script once and then changed the computer’s clock to be a Wednesday, and re-ran the script. We then changed the day to a Sunday and ran it once more.

./if-shop.sh

./if-shop.sh

./if-shop.sh

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The case For if

Conditional execution is what brings power to programming and scripting, and the humble if statement might well be the most commonly used way of switching the path of execution within code. But that doesn’t mean it’s always the answer.

Writing good code means knowing what options you have and which are the best ones to use in order to solve a particular requirement. The if statement is great, but don’t let it be the only tool in your bag. In particular, check out the case statement which can be a solution in some scenarios.

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