Shell – Why is Brace Expansion Not Supported?

brace-expansionshell

Do I have to do any configuration to start brace expansion? When I run mkdir {1..10}, it just creates a dir naming {1..10}.

Best Answer

While brace expansion like {1,2} originates in csh in the late 70s, and found its way to Bourne-like shells in bash/zsh/pdksh in the late 80s, early 90s, the {n1..n2} variant came later first in zsh in 1995 (2.6-beta4).

bash copied it in 2004 (3.0) and ksh93 in 2005 (ksh93r).

Probably the shell you're trying this in is neither of those or is an older version of bash and ksh93.

Related Solutions

Bash – When do you use brace expansion

Brace expansion is very useful if you have long path names. I use it as a quick way to backup a file:

cp /a/really/long/path/to/some/file.txt{,.bak}

will copy /a/really/long/path/to/some/file.txt to /a/really/long/path/to/some/file.txt.bak

You can also use it in a sequence. I once did so to download lots of pages from the web:

wget http://domain.com/book/page{1..5}.html

for i in {1..100}
do
   #do something 100 times
done

Bash – Brace expansion does not work for less than 2 arguments

Yes, n > 1 is an explicit requirement:

A correctly-formed brace expansion must contain unquoted opening and closing braces, and at least one unquoted comma or a valid sequence expression. Any incorrectly formed brace expansion is left unchanged.

As for the why - historical reasons, to some extent (though it was copied from csh originally, which has the other behaviour). There are commands that take {} as a literal argument (find, parallel, and others with more complex arguments), and also other uses of {} in the shell language. Because brace expansions are only processed when written literally (and not from variables), there's really no motivation to support degenerate expansions, and some reasons not to.

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Related Solutions

Bash – When do you use brace expansion

Bash – Brace expansion does not work for less than 2 arguments

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