Alright, here’s a quick trick that I use at least a few times a week.

Problem

Let’s say we have a file that we want to make a copy of for whatever reason. In this example we’re creating a backup of the SSH server config before modifying it:

$ cp /etc/ssh/sshd_config /etc/ssh/sshd_config.bak

This is fine and dandy, but a bit verbose… We’re writing the same string two times and to be frank, it’s not fun.

Solution

Bash has a feature called brace expansion.

The snippet below is equivalent with the one up above:

$ cp /etc/ssh/sshd_config{,.bak}

and if we want to overwrite the active file (sshd_config) with the backed up one, we can do the reverse operation (note the placement of the comma):

$ cp /etc/ssh/sshd_config{.bak,}

which would expand to, yes you guessed it:

$ cp /etc/ssh/sshd_config.bak /etc/ssh/sshd_config

Other use cases

While where at it, here are some other useful expansions.

  1. Create header and source files for the app, widget and parser classes
    $ touch src/{app,widget,parser}.{h,cpp}
  2. Download img-1.jpg through img-10.jpg
    $ wget https://acme.com/images/img-{1..10}.jpg
  3. Move all nine files to another directory
    $ mv file-{a..c}{1..3}.txt some-dir/

References

See bash(1) EXPANSION.

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