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xargs(1)

Published: Mar 2021
Updated: Mar 2021

xargs - build and execute command lines from standard input

xargs translates arguments to commands. This effectively means you can run one command per argument, or forward all arguments to one command . Here’s some examples to make it stick. We’ll cover the finer details later.

# Stop all running Docker container ids
$ docker ps -q | xargs docker stop
# Get logs for all running docker containers
$ docker ps -q | xargs -L1 docker logs

These two examples demonstrate the common uses. The first commands take lists the all running Docker container ids and forwards them to docker stop. Here’s what this looks like in a verbose mode:

$ docker ps -q | xargs -t docker stop
docker stop 23489abc8ac 891bcadea134

The -t option instructs xargs to print the command before executing it. xargs essentially looped over all arguments. Apply -t to the second example to see the difference:

$ docker ps -q | xargs -t -L1 docker logs
docker logs 891bcadea134
docker logs 23489abc8ac

The -L option controls the number of arguments applied to the given command. xargs -L1 executes one command per argument. xargs without -L executes one command with all arguments. Naturally you can control the number invocations by changing the -L option.

Use Case

Use xargs when you need to run a single command against multiple arguments. Do not use xargs if you need to execute logic against each argument (such as if x, then y) or work with pipes. Use while loop in that case. xargs is essentially your one-liner chainsaw! Use without -L when the command supports multiple arguments (e.g the command processes all arguments given). Use with -L1 when the command accepts a single argument.

Signature & Key Options

xargs [options] [command [initial-arguments]]

xargs’s signature is straight forward. It does not very based on options. We already discussed -L to control executions and -t to print the command before running it. These two options cover 80% of use cases in my experience. Onto the other 20%.

xargs drops the arguments onto the end of command by default. So what do you do if you need to apply options/arguments after the argument? You use -I.-I provides the “replacement string”. This is effectively a template variable in the argument list.

Here’s an example using the service command. service looks like this: service NAME ACTION. NAME and ACTION are required. Here’s an example: service http stop. You could not use xargs directly with servicebecause NAME is not the last arguments. You can with -I.

# assume a list of names in services.txt
$ cat services.txt | xargs -L1 -I {} service {} stop

Here {} is the replacement variable. xargs replaces all {} occurrences in the arguments. -I has a hidden gotcha. The value should not be a shell control character (which vary per shell; hint: fish)! You could not use # because that’s a comment character, nor would you use | since that’s for redirects. Use {} (by convention) in sh/bash/zh or % in fish.

-I replaces the -R number of occurrences. -R defaults to. Read manual on -J for an alternate replacement approach.

xargs supports parallel execution via -P. This is easier than working with parallel. Combine with $(nproc) to execute the correct number of commands for machine’s processor.

BSD vs GNU vs Busybox

xargs varies slightly (and annoyingly so) between BSD, GNU, and Busybox. They share the same signature, but support different options.

  • GNU xargs supports a -r / --no-run-if-empty. This is useful when argument list (perhaps generated dynamically) may be empty. GNU xargs exits successfully when this option and an empty argument list. BSD xargs does not support this option and fails if the argument list is empty.
  • BSB xargs does not have --verbose. BSD uses -t, while GNU supports -t and --verbose.
  • GNU xargs supports -p / --interactive to confirm individual commands before execution.
  • BSD xargs does not support long options.
  • Both versions run command against all arguments. Neither have an option to stop if command fails on one argument.
  • Busybox xargs uses -n instead of -L

Protips

  • Use -I @. This is a single character that does not need escaping and works across all shells (Fish included). Don’t believe the {} hype.