Operation and Maintenance
Linux Operation and Maintenance
How to check if a command succeeded in bash
Checking whether the command is successful in Bash can be achieved by exiting the status code. 0 indicates success, non-zero value indicates error; use $? to view the exit code of the previous command, such as ls /some/directory; echo $?; or it can be directly judged in the if statement, such as if mycommand --option; then echo "Success"; else echo "Failed"; fi; Common exit codes include 0 (success), 1 (general error), 2 (incorrect command usage), etc.; use set -e to make the script exit immediately when any command fails, but be used with caution to avoid misjudgment of non-serious errors.
In Bash, checking whether a command succeeded is straightforward: you look at its exit status. By default, a command returns 0 if it succeeded and a non-zero value (usually 1 , but sometimes others) if something went wrong.
Here's how to check it in your scripts or on the command line.
Using $? to check the exit code
After running any command in Bash, you can immediately check the special variable $? . It holds the exit status of the last command executed.
For example:
ls /some/directory echo $?
- If
lsfound the directory and listed it, you'll see0. - If the directory doesn't exist or there was a permission issue, you'll get a number like
2.
This method works well in scripts when you want to conditionally do something based on success or failure.
A simple usage in a script:
#!/bin/bash mycommand --option if [ $? -eq 0 ]; then echo "Command succeeded" else echo "Command failed" fi
Just remember — once another command runs after the one you care about, $? gets overwritten. So use it right away.
Using if directly with the command
Instead of using $? , you can put the command directly inside an if statement. This is cleaner and avoids possible mistakes from forgetting that $? changes quickly.
Here's how that looks:
if mycommand --option; then echo "Success!" else echo "Failed!" fi
This is often preferred because:
- It's more readable.
- You don't have to manually compare numbers.
- Less chance of accidentally using the wrong
$?.
You can also chain this with other logic, like retrying a command or logging errors.
Common exit codes and what they mean
Exit codes are defined by each command, but some are standard:
-
0= Success -
1= General error -
2= Misuse of shell builtins (like incorrect options) -
126= Command found but not executed -
127= Command not found -
130= Script interrupted (eg, Ctrl C pressed)
These aren't always consistent across all programs, but they give a general idea of ??what went wrong.
If you're debugging a script or trying to understand why something didn't work, checking these values ??can help pinpoint the issue.
Use set -e to exit on error in scripts
If you're writing a script and want it to stop as soon as any command fails, add this at the top:
#!/bin/bash set -e
This tells Bash to exit immediately if any command exits with a non-zero status. It's useful for automation where partial execution might leave things in an inconsistent state.
But be careful — sometimes commands return non-zero even when it's not a serious problem. For example, grep returns 1 if it finds no matches, which isn't always an error.
So, use set -e wisely, especially if you're relying on certain commands to run even if previous ones fail.
That's basically it. Checking if a command succeeded in Bash comes down to understanding exit codes and knowing how to access or act on them — either through $? , direct conditions, or script flags like set -e .
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