Today I faced an odd problem. I needed to remove everything but a specific file from a directory via the command line. Having thought about it for a while, and done a bit of research, I came across someone posing the same question on Stack exchange. There are several answers provided, but I really liked this one.

Example

ls | grep -v file.txt | xargs -t rm

In this example, everything but file.txt will be removed from the working directory.

How it works

  1. The output from the ls command is piped into grep
  2. grep is passed the -v argument which inverts its behaviour, returning everything but file.txt
  3. The output of grep is then piped into xargs which in turn passes each result to rm (which removes each item passed to it). Note: the -t argument simply shows the resulting command that is run by xargs

I thought that was a nice simple way of achieving what I needed and, since it also neatly demonstrates how piping and xargs works, I thought I’d post it here.

Using xargs with commands that take two arguments

But what about using xargs for commands that take multiple arguments (like mv that takes both source and destination arguments)? I used this command today to move all Python files within the current directory to a new location.

ls | grep -e '.*py$' | xargs -I '{}' mv '{}' ../python

How it works

  • The output of ls is piped to grep
  • the -e argument is passed to grep to use extended regular expressions
  • the output is passed to xargs with the -I argument using '{}' to represent the replacement.

I should explain that I initially found this confusing because it looked to me that standard input was being placed both before and after the mv command. That isn’t the case because the first instance of '{}' is simply providing a name for the value received from standard input. I’ve experimented with this and you could achieve the exact same thing with something other than '{}'. Here’s an example where I’m copying files using '{blah}':

ls | grep .py | xargs -I {blah} cp {blah} new-{blah}