Oh the terminal, we are all familiar with the Linux terminal. Maybe a little too when. When executing commands in the Linux terminal, you may come across instances where you need to determine whether a command executed successfully.
This is especially useful in automated commands such as bash scripts or cron jobs where the execution of a previous command can impact the execution of the rest of the program.
Checking If a Command Succeeded in Unix-like Systems
Unix-like systems such as Linux and MacOS, use a mechanism called exit status to indicate whether a command has succeeded or not.
The exit status is an integer value where 0 indicates success, and any other number between 1 and 255 indicates a failure. This status is stored in a special variable known as $?
Consider the examples below that demonstrates the usage of this variable.
touch test.txt
echo $?
In the above example, the touch command creates a new file named test.txt.
We then use the echo $? command then prints out the exit status of the touch command. If the file is successfully created, echo $? will output 0.
This mechanism can be very useful in shell scripts. For instance, you can perform different actions depending on whether a command succeeds or fails.
Consider the example shell script that uses the $? variable:
#!/bin/bash
touch test.txt
# Check if it succeeded
if [ $? -eq 0 ]
then
echo "The command succeeded."
else
echo "The command failed."
fi
In the example, we start by attempting to create a file with the specified name. If the command succeeds, it will print the message corresponding to the error.
As you can guess, you can expand this feature to ensure more complex logic and error-handling mechanisms.
Conclusion
In this tutorial, we explored how you can check whether a previous command executed successfully in Linux and other Unix-base systems.
