Bash Brace Expansion

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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.

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Seems to not work for the "host" command

Samuel Huckins's picture

I love brace expansion, but have found it doesn't work with the host command for some reason:

# Normal
~: echo {1,2}
1 2

# Individual commands with host
~: host www2.yahoo.com
www2.yahoo.com is an alias for rc.yahoo.com.
rc.yahoo.com is an alias for rc.fy.b.yahoo.com.
rc.fy.b.yahoo.com has address 206.190.60.37

~: host www1.yahoo.com
www1.yahoo.com is an alias for rc.yahoo.com.
rc.yahoo.com is an alias for rc.fy.b.yahoo.com.
rc.fy.b.yahoo.com has address 206.190.60.37

# But as soon as I add brace expansion...
~: host www{1,2}.yahoo.com
;; connection timed out; no servers could be reached

Any ideas?

It works but it doesn´t...

Mitch Frazier's picture

Brace expansion is working, the problem is that host doesn't like the arguments you're giving it. Check the man page for host:

SYNOPSIS
    host [OPTIONS] {name} [server]

DESCRIPTION
    ...

    name is the domain name that is to be looked up. ...

    server is an optional argument which is either the name or IP address
    of the name server that host should query instead of the server or servers
    listed in /etc/resolv.conf.

So when you do:

  host www1.yahoo.com www2.yahoo.com

you're telling host to use www2.yahoo.com as the DNS server. The crux is that host only looks up a single name per command invocation.

Mitch Frazier is an Associate Editor for Linux Journal.

New feature for increment in Bash V4

Philippe Petrinko's picture

Hello Mitch,

Nice topic, thanks for your work, I enjoy LinuxJournal.

According to those 2 pages,
http://bash-hackers.org/wiki/doku.php/syntax/expansion/brace
http://www.gnu.org/software/bash/manual/bashref.html#Brace-Expansion

Increment has been added to the brace expansion in Bash v4.

Do you agree?

Philippe Petrinko (Versailles, France)

I Agree That They So

Mitch Frazier's picture

I have not used bash v4.0 so I can't confirm that the feature exists. I assume that they're not must making it up, although I will note that the news file, which according to the bash home page tersely lists the new features in bash-4.0, does not make any mention of the feature.

Mitch Frazier is an Associate Editor for Linux Journal.

increment

Anonymous's picture

Hi,

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

No increment

Mitch Frazier's picture

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.

Possible in bash 4

Marcel Grünauer's picture

In bash 4, you can specify an increment:

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

The second example is not

Anonymous's picture

The second example is not quite right:

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

Oops

Mitch Frazier's picture

Typo, thanks, I'll fix it.

Mitch Frazier is an Associate Editor for Linux Journal.

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