Unix tip #1: advanced mkdir and brace expansion fun

If you don’t know all the ins and outs of the mkdir command, you are probably expending more effort than necessary.  Imagine this fairly common use case:

You are in a folder and want to create one folder which has 3 sub folders.  Let’s call the main folder Programming and its 3 sub folders Java, Python, and Scala.  Visually this looks like

or rendered via tree:

Programming/
|-- Java
|-- Python
`-- Scala

A first pass at accomplishing this would be to create the Programming folder, and then the three individual folders underneath

$ mkdir Programming
$ mkdir Programming/Java
$ mkdir Programming/Python
$ mkdir Programming/Scala

This certainly works, but it takes four commands.

Let’s see if we can’t do better.  Delete those folders with the command

rm -rf Programming/

This will delete the programming folder and all subfolders underneath it (the r flag is for recursive, the f flag for forcing the removal of nonempty directories)

Like most unix commands, the mkdir command can take multiple arguments, separated by spaces.  So the three separate commands to create Java, Python, and Scala can be put onto one line.

mkdir Programming; mkdir Programming/Java Programming/Python Programming/Scala

Note the ; separator between the two commands.  We need to create the Programming folder before we can create the subfolders.

This is better but still too verbose.  It would be nice to remove the mkdir Programming call; we’d like to be able to create an arbitrarily nested folder and have mkdir create all the parent folders automatically.  Fortunately there is a way to do this: the -p flag of mkdir does exactly this.

 -p      Create intermediate directories as required.  If this option is not specified, the full path prefix of
             each operand must already exist.  On the other hand, with this option specified, no error will be
             reported if a directory given as an operand already exists.  Intermediate directories are created with
             permission bits of rwxrwxrwx (0777) as modified by the current umask, plus write and search permission
             for the owner.

Thus we can change our command to

mkdir -p Programming/Java Programming/Python Programming/Scala

This is better but still not perfect; we’re repeating ourselves 3 times with the Programming call.  Enter an absurdly useful Bash shell construct known as brace expansion.

echo {5,6,7}
5 6 7

The arguments within braces are treated as if they were space separated instead.  That wouldn’t be terribly useful except that things immediately before the brace are repeated as well

echo hello{5,6,7}
hello5 hello6 hello7

This brace expansion can be used anywhere, since the textual substitution happens before the arguments are passed into other processes.  So, combining this with what we saw earlier, we can put Java, Python and Scala into a list and prepend it with Programming:

 echo Programming/{Java,Python,Scala}
Programming/Java Programming/Python Programming/Scala

That should look very familiar.  Putting it in place of the earlier mkdir command we get the elegant one liner

mkdir -p Programming/{Java,Python,Scala}

Certain versions of bash also support numerical ranges within the brackets:

echo {1..10}
1 2 3 4 5 6 7 8 9 10

Conclusion:

I have shown you how to create all the parent directories using the mkdir command, and introduced you to the brace expansion macro of Bash.  The latter is extremely powerful, and can be used to great effect within scripts.

Note: The arguments within the braces must have NO space between after or before the commas in order for the brace expansion to work.

[572][nicholasdunn: Desktop]$ echo {5, 6, 7}
{5, 6, 7}
[573][nicholasdunn: Desktop]$ echo {5,6,7}
5 6 7

Advertisement