Bash Brace Expansion

The new tricks Bash has picked up in 3.0 are exciting and useful, but simple brace expansion has been available for some time now, and yet we seldom see it used. I suppose that's because the need doesn't come up too often. The classic example from the Bash man page is:

mkdir /usr/local/src/bash/{old,new,dist,bugs}
 

which expands to

mkdir /usr/local/src/bash/old /usr/local/src/bash/new \
   /usr/local/src/bash/dist /usr/local/src/bash/bugs
 

The man page also gives a more complex example showing nested braces:

chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
 

That's a little harder to follow, but if you replace "chown root" with "echo" you can see it:

$ echo  /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
/usr/ucb/ex /usr/ucb/edit /usr/lib/ex?.?* /usr/lib/how_ex
 

There are other possibities beyond nesting:

$ echo foo{1,2}{a,b}
foo1a foo1b foo2a foo2b
 

And did you know that you can leave out the arguments? As long as you have the comma, this works:

$ echo foo{,}{,}
foo foo foo foo
 

And so does this:

$ echo foo{,,,}
foo foo foo foo
 

Braces can get pretty complex:

$ echo foo{1,2,3,4}bar{7,8{foo,bar}}
foo1bar7 foo1bar8foo foo1bar8bar foo2bar7 foo2bar8foo foo2bar8bar foo3bar7 
foo3bar8foo foo3bar8bar foo4bar7 foo4bar8foo foo4bar8bar
# with 3.0 bash the "foo{1,2,3.4}" can be "foo{1..4}"
 

But when would you ever need anything like that?

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4 comments

Mon Apr 21 22:38:23 2008: 4091   KenGregg


Cool. I guess that'll teach me to not RTFM cover to cover.

Back in my early days I worked with an os where the shell would expand what they referred to as an @list. Create a list of file names in a file and it would be expanded to the list with @filename syntax.

I have often wished I could do something like that in bash. I'll have to play with this.



Mon Apr 21 22:40:45 2008: 4092   TonyLawrence


If you read everything cover to cover you'll never get anything else done :-)



Tue Apr 22 01:02:57 2008: 4097   badanov


Works in sh as well. Maybe bash got this working from the sh shell?



Tue Apr 22 13:05:11 2008: 4108   BigDumbDinosaur


mkdir -p <path> produces the (more or less) equivalent to bash's braces. However, a separate <path> has to be specified for each subdirectory that is desired, so there's a bit more typing involved.

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