Bash Brace Expansion

May 30th, 2008 by Mitch Frazier in

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Bash brace expansion is used to generate stings at the command line or in a shell script. The syntax for brace expansion consists of either a sequence specification or a comma separated list of items inside curly braces "{}". A sequence consists of a starting and ending item separated by two periods "..".

Some examples and what they expand to:

  {aa,bb,cc,dd}  => aa bb cc dd
  {0..12}        => 0 1 2 3 4 5 6 7 8 9 10 11 12
  {3..-2}        => 3 2 1 0 -1 -2
  {a..g}         => a b c d e f g
  {g..a}         => g f e d c b a
If the brace expansion has a prefix or suffix string then those strings are included in the expansion:
  a{0..3}b       => a0b a1b a2b a3b
Brace expansions can be nested:
  {a,b{1..3},c}  => a b1 b2 b3 c

Counted loops in bash can be implemented a number of ways without brace expansion:

# Three expression for loop:
for (( i = 0; i < 20; i++ ))
do
    echo $i
done
# While loop:
i=0
while [[ $i -lt 20 ]]
do
    echo $i
    let i++
done
# For loop using seq:
for i in $(seq 0 19)
do
    echo $i
done
A counted for loop using bash sequences requires the least amount of typing:
for i in {0..19}
do
    echo $i
done
But beyond counted for loops, brace expansion is the only way to create a loop with non-numeric "indexes":
for i in {a..z}
do
    echo $i
done

Brace expansion can also be useful when passing multiple long pathnames to a command. Instead of typing:

  # rm /a/long/path/foo /a/long/path/bar
You can simply type:
  # rm /a/long/path/{foo,bar}

Brace expansion is enabled via the "set -B" command and the "-B" command line option to the shell and disabled via "set +B" and "+B" on the command line.

__________________________

Mitch Frazier is an Associate Editor for Linux Journal and the Web Editor for linuxjournal.com.


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Anonymous's picture

increment

On April 15th, 2009 Anonymous (not verified) says:

Hi,

Is it possible to specify an increment in the brace expansion?

Mitch Frazier's picture

No increment

On April 16th, 2009 Mitch Frazier says:

You mean something like:

   a{0..4:2}b  => a0b a2b a4b

If so, the answer is no.

__________________________

Mitch Frazier is an Associate Editor for Linux Journal and the Web Editor for linuxjournal.com.

Anonymous's picture

The second example is not

On May 30th, 2008 Anonymous (not verified) says:

The second example is not quite right:

a{0..3}b => a0b a1b a2b a3b

Mitch Frazier's picture

Oops

On May 31st, 2008 Mitch Frazier says:

Typo, thanks, I'll fix it.

__________________________

Mitch Frazier is an Associate Editor for Linux Journal and the Web Editor for linuxjournal.com.

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