How to Use the Command 'mispipe' (with Examples)

‘Mispipe’ is a handy utility from the ‘moreutils’ package that allows you to pipe two commands and return the exit status of the first command in the pipeline. Typically, in Unix pipelines, the exit status returned by the shell corresponds to the last command in the pipeline. ‘Mispipe’ alters this behavior to return the exit status of the first command, which can be crucial in some troubleshooting and scripting scenarios.

Use case 1: Pipe Two Commands and Return the Exit Status of the First Command

Code:

mispipe "grep 'error' somefile.log" "sort"

Motivation:

In this scenario, suppose you are analyzing a log file named somefile.log to check for occurrences of the string “error.” By using grep to filter these lines, you can then pipe the results to the sort command to organize them alphabetically or numerically. Still, your primary interest is in knowing if the ‘grep’ command itself was successful, i.e., whether it found any errors. With traditional piping, you’d get the exit status of sort, which is likely irrelevant for this use case since sorting is almost always successful as long as it receives input.

Explanation:

• mispipe: Initiates the process by signaling that you wish to pipe commands and get the status of the first one.

• grep 'error' somefile.log: This part of the command is the first command in the pipeline. It searches for lines containing the word ’error’ in the file somefile.log.

• sort: This is the second command in the pipeline, which will sort the lines output by the grep command. Although it processes the lines, the success or failure of sort isn’t our primary concern here.

Example Output:

If the command grep successfully finds lines with “error,” mispipe will return a resulting exit status of 0. If no errors are found, grep by default returns 1, and mispipe will carry this status out.

1

This output indicates that the ‘grep’ command did not find any lines containing “error” in somefile.log.

Conclusion

The mispipe command is a simple yet powerful utility for instances where the success of the first command in a UNIX pipeline is critical. By allowing users to obtain the exit status of the first command rather than the last, mispipe can provide substantial benefits in debugging and scripting. This ability is particularly useful in situations where the success of preparatory commands determines the next steps in data processing or system maintenance scripts, allowing for more precise flow control and error management.