How to Use Double Pipe “||” in Bash [3 Examples]

The double pipe (‘||’) operator in Bash scripting is a versatile tool that enables conditional execution. It allows you to execute a command or a series of commands only if the preceding command returns a non-zero exit status, indicating a failure. In this article, I will show you how to use double pipe “||” in Bash scripts with 3 practical examples.

What is Double Pipe (||) in Bash?

The double pipe (||) is a logical OR operator used in various programming and scripting languages, including Bash. It is used to combine two or more conditions or commands and returns true if at least one of them is true.

In a more simple context, every process returns an exit status (either “0” or “non-zero”). A code of 0 means the process ran successfully while a non-zero indicates an error. And, a double pipe causes commands to be executed only if the preceding command returns a non-zero exit status. You can understand the process even better if you look at it through the Truth Table of the OR operator:

If Command 1 is true the result will always be the output of Command 1, regardless of the value of Command 2. And, if Command 1 is false, the result will be the output of Command 2.

Now, if I explain the basic syntax of using the double pipe (||) in commands, then I could just simply write:

<Command 1> || <Command 2>

Here, if Command 1 succeeds (returns a zero exit status), then Command 2 will not be executed. And, if Command 1 fails (returns a non-zero exit status), then Command 2 will be executed.

Check out this simple example of using ‘Double Pipe’:

Suppose you want to create a backup of a file and be sure it was successfully backed up. You can use the double pipe operator for a situation like this:

cp file.txt backup_file.txt  || echo “Backup Failed”

See from the image, as the cp command fails to create the backup, the echo command displays Backup Failed.

3 Examples of Using Double Pipe “||” in Bash Scripts

Using the double pipe operator ‘||’ for simple operations in a shell script is a much easier approach to handling errors and failures more effectively. While other methods (i.e. if-else) might be verbose with increased code length & prone to more errors, double pipe allows for concise, one-liner error-handling actions when a specific command fails. In the following article, I will explain how the operator works with practical Bash script examples, so read along:

1. Create a Directory If it Doesn’t Exist

You can test the existence of a directory using a conditional expression [ condition ], where the conditional test operator -d checks if the directory with the same name stored in a variable exists or not. And, if this file doesn’t exist (returns a non-zero exit status), the next part of the double pipe || will be executed & a new directory with the name will be created.

Here’s how to create a directory if it doesn’t exist using double pipe in Bash:

#! /bin/bash

directory_name="test"
[ -d "$directory_name" ] || mkdir "$directory_name"

The script checks if the directory “test” exists, and if it doesn’t, it creates a same-named directory using the mkdir command.

Now, let’s see if the directory ‘test’ exists or not by running the script: The test directory did not exist in my system, & from the output of the ls -l command, you can see it was created (recently after I ran the script).

2. Install a Package If Not Already Installed

Suppose you need to install a package but not if it is already installed. In that case, you can create a conditional script using the || logical operator that will search for the package (if installed) in your system & if the result is non-zero then it will install the package.

Use the following script to install a package if it’s not already installed:

#! /bin/bash

package_name="curl"
dpkg -l | grep -q "ii  $package_name" || sudo apt install "$package_name"

This script will quietly search (grep command with the option –quiet) the package name from the installed packages (represented by ii) from the output of the dpkg -l command. If the package is installed the second command sudo apt install $package name will not run, otherwise, it will install the package.

Let’s check if the curl command line tool exists in my system or needs to be installed by running the script:No output means the package (curl) was already installed.

3. Delete a File If it Exists

Sometimes you need to create a file but you’re not sure if any file with the same name already exists in your system & you just want to create a new one with that name. You can do that just by deleting the existing ones.

For that, first, check the existence of the file with the test operator -e & if this part is true (returns ‘0’ exit status), run the next part of the command using the && operator (executes the command only if the preceding one returns true).

To delete a file if it exists using Bash script, use the following code snippet:

#! /bin/bash

file_name="input.txt"
[ -e "$file_name"  ] && rm "$file_name" || echo "File not found: $file_name"

The script defines a variable with the file name to be checked as value & -e checks the file’s existence. If it exists, then the rm command removes it & as the first two parts of the commands will be executed the last part (after the || operator) won’t get executed. But, if the first two-part returns a non-zero value, the last part will be executed & display File not found: $file_name.

Now, let’s check if the file input.txt exists in my system by running the script: The file input.txt wasn’t removed as it doesn’t exist in my system.

Conclusion

To sum up, the double pipe (||) operator is a simple yet powerful tool for conditional execution in Bash scripts. By understanding how it works & combining it with other operators, you can create much more effective & flexible scripts. Hope this writing helps you to understand the operator perfectly!

People Also Ask

What Does “||” in Bash Mean?

In Bash, the double-pipe || symbol is a logical operator used for the conditional execution of commands. Specifically, it is used to execute a command only if the command preceding it fails. This means if a command on the left side of || succeeds (exits with a status code of 0), the command on the right side of || will not be executed & vice versa.

What is a Pipe in Bash?

A ‘pipe’ is a technique used in Bash & other Unix-like shell environments for connecting the output of one command to the input of another command. It is represented by the vertical bar symbol |. The simple syntax for using pipe between 3 commands is, Command 1 | Command 2 | Command 3.

What is the Difference Between a Single Pipe and a Double Pipe?

The single pipe | is used for creating pipelines that allow the output of one command to be passed as input to another command, more like for chaining commands together. Whereas, the double pipe || is a logical OR operator used in conditional statements, in a simple context, to execute the second command only if the first command fails.