xargs command: turn a list into arguments

Syntax

command that lists things | xargs [options] command to run

It rearranges piped input into arguments for the next command.

Try it first

Some commands read standard input; others only take arguments. rm is the second kind.

$ find tmp -name '*.tmp'
tmp/cache1.tmp
tmp/cache2.tmp
$ find tmp -name '*.tmp' | rm
rm: missing operand

The list arrived, but rm never looked at it. With xargs in between, the list becomes arguments.

$ find tmp -name '*.tmp' | xargs rm
$ ls tmp

Gone. That is the whole job of xargs.

All at once

By default everything is handed over in one run.

$ ls logs
access.log  app.log  error.log
$ ls logs | xargs echo
access.log app.log error.log

Which makes counting the results of a find completely natural.

$ find logs -name '*.log' | xargs wc -l
3 logs/app.log
2 logs/access.log
1 logs/error.log
6 total

Paired with grep -l, which prints only the names of matching files, you can act on just the files that contain an error.

$ grep -l ERROR logs/*.log | xargs wc -l
3 logs/app.log
1 logs/error.log
4 total

One at a time

-n 1 runs the command once per value.

$ echo 'a b c' | xargs -n 1 echo
a
b
c

Choosing where the value goes

When the value must go somewhere other than the end, use -I.

$ find logs -name '*.log' | xargs -I {} cp {} backup/
$ ls backup
access.log  app.log  error.log

Each value is substituted at {}. Commands like cp, where the order of source and destination is fixed, need this form.

$ ls logs | xargs -I {} echo 'file: {}'
file: access.log
file: app.log
file: error.log

When you actually reach for it

xargs is what you type to feed what you found into what you want done.

Things that trip people up

Check with echo before deleting. xargs rm asks nothing. Swapping in xargs echo and reading the list first is the habit worth having.

Spaces in file names break it. xargs splits on whitespace, so my file.txt arrives as two arguments. -I {} takes the whole line as one value and avoids that.

An empty list still runs the command once.

$ find . -name 'nope*' | xargs rm
rm: missing operand

With no matches, rm runs with no arguments at all. One more reason to try destructive pipelines with xargs echo first.

Commands that read standard input do not need it. grep and sort read input themselves. Putting xargs in front turns that input into arguments, which means something different.