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2010年05月28日

bash brace expansion

Last time we looked at bash parameter expansion. Now I would like to talk about bash brace expansion. This is mainly a blog about Perl, but Unix tools like shells and editors are part of a Perl programmer's life as well, so from time to time I'll write about those tools too.

Brace expansion in bash is a way of generating strings. The most general form is a list of strings within braces, separated by commas. There can be other strings before and/or after the braces; they will be included in all variations. For example:

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

$ echo prefix-{foo,bar,baz}-suffix
prefix-foo-suffix prefix-bar-suffix prefix-baz-suffix

$ for i in {foo,bar,baz}; do echo $i; done
foo
bar
baz

Brace expansion can be turned on and off using bash's B option. +B turns brace expansion off, -B turns it back on.

$ set +B

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

$ set -B

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

Here are some easy examples that show what is possible:

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

$ echo {1..3}{1..3}{1..3}
111 112 113 121 122 123 131 132 133 211 212 213 221 222
223 231 232 233 311 312 313 321 322 323 331 332 333

$ echo {foo,bar,baz}{1..3}
foo1 foo2 foo3 bar1 bar2 bar3 baz1 baz2 baz3

$ echo {foo{1..3},bar{4..6},baz{7..9}}
foo1 foo2 foo3 bar4 bar5 bar6 baz7 baz8 baz9

$ cp /some/very/long/path/to/my.conf{,.bak}
cp /some/very/long/path/to/my.conf /some/very/long/path/to/my.conf.bak

$ mkdir -p /Foo-Bar/{bin,lib,eg,t}
mkdir -p /Foo-Bar/bin /Foo-Bar/lib /Foo-Bar/eg /Foo-Bar/t

$ mv foo{,-}bar
mv foobar foo-bar

You can also specify ranges using the {from..to} syntax, and ranges with steps using the {from..to..step} syntax. These ranges can be numeric or alphabetic.

$ echo {1..5}
1 2 3 4 5

$ echo {5..1}
5 4 3 2 1

$ echo {3..-3}
3 2 1 0 -1 -2 -3

$ echo {1..10..2}
1 3 5 7 9

$ echo {a..e}
a b c d e

$ echo {e..a}
e d c b a

$ echo {a..z..2}
a c e g i k m o q s u w y

One typical use for brace expansion is to tell wget or curl to download several files whose URLs conform to a certain pattern:

$ wget http://example.com/path/to/{014..017}.{html,png}
wget http://example.com/path/to/014.html http://example.com/path/to/014.png
http://example.com/path/to/015.html http://example.com/path/to/015.png
http://example.com/path/to/016.html http://example.com/path/to/016.png
http://example.com/path/to/017.html http://example.com/path/to/017.png

(bash version 4 retains leading zeros; earlier bash versions omitted them.)

I also use brace expansion in my ~/.bashrc to inspect certain directories to find out whether to they contain bin/, sbin/ or man/ subdirectories that should be added to $PATH and $MANPATH:

for d in {/usr,/opt,~}{,/{local,share,local/share,perl,perl/5.*.*}}
do
    test -d "$d/bin" && PATH="$d/bin:$PATH"
    test -d "$d/sbin" && PATH="$d/sbin:$PATH"
    test -d "$d/man" && MANPATH="$d/man:$MANPATH"
done

This effectively iterates over all of these directories:

/usr
/usr/local
/usr/share
/usr/local/share
/usr/perl
/usr/perl/5.*.*
/opt
/opt/local
/opt/share
/opt/local/share
/opt/perl
/opt/perl/5.*.*
~
~/local
~/share
~/local/share
~/perl
~/perl/5.*.*

Do you have interesting examples of using the bash brace expansion mechanism?

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posted at: 10:12 | path: /dev | permalink | 1 comment | 0 trackbacks

Justin Davis wrote at 2010-05-29 08:49:

I always thought the brace expansion was a neat and unique feature of bash. Earlier today I found out perl can do it too! It's in perl's glob function (which should also work inside <>'s) like so:

print "$_\n" for glob '{/usr,/opt,~}{,/{local,share,local/share,perl,perl/5.\*.\*}}';

(One problem is you have to escape the *'s with \'s. If they were in double quotes you'd need two \\s.)

I use brace expansion wherever I can but that one in your bashrc is longer than any one I've used, impressive! I didn't know you could do ranges in bash that is cool!

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