Bash Brace Expansion
May 30th, 2008 by Mitch Frazier in
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
for i in {0..19}
do
echo $i
done
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/barYou 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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Seems to not work for the "host" command
On October 20th, 2009 Samuel Huckins (not verified) says:
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...
On October 21st, 2009 Mitch Frazier says:
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:
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 and the Web Editor for linuxjournal.com.
New feature for increment in Bash V4
On July 16th, 2009 Philippe Petrinko (not verified) says:
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
On July 16th, 2009 Mitch Frazier says:
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 and the Web Editor for linuxjournal.com.
increment
On April 15th, 2009 Anonymous (not verified) says:
Hi,
Is it possible to specify an increment in the brace expansion?
No increment
On April 16th, 2009 Mitch Frazier says:
You mean something like:
a{0..4:2}b => a0b a2b a4bIf so, the answer is no.
__________________________Mitch Frazier is an Associate Editor for Linux Journal and the Web Editor for linuxjournal.com.
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
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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