Brace expansion [Bash Hackers Wiki]

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====== Brace expansion ======

{{keywords>bash shell scripting expansion substitution text list brace}}

<code>
{string1,string2,...,stringN}
{<START>..<END>}

{<START>..<END>..<INCR>} (Bash 4)

<PREFIX>{........}

{........}<SUFFIX>

<PREFIX>{........}<SUFFIX>
</code>

Brace expansion is used to generate arbitrary strings. The specified strings are used to generate **all possible combinations** with the optional surrounding prefixes and suffixes.

Usually it's used to generate mass-arguments for a command, that follow a specific naming-scheme.

:!: It is the very first step in expansion-handling, it's important to understand that. When you use
<code>
echo {a,b}$PATH
</code>
then the brace expansion **does not expand the variable** - this is done in a **later step**. Brace expansion just makes it being:
<code>
echo a$PATH b$PATH
</code>
Another common pitfall is to assume that a range like ''{1..200}'' can be expressed with variables using ''{$a..$b}''. Due to what I described above, it **simply is not possible**, because it's the very first step in doing expansions. A possible way to achieve this, if you really can't handle this in another way, is using the ''eval'' command, which basically evaluates a commandline twice: <code>eval echo {$a..$b}</code> For instance, when embedded inside a for loop : <code>for i in $(eval echo {$a..$b})</code> This requires that the entire command be properly escaped to avoid unexpected expansions. If the sequence expansion is to be assigned to an array, another method is possible using [[commands:builtin:declare|declaration commands]]: <code>declare -a 'pics=(img{'"$a..$b"'}.png)'; mv "${pics[@]}" ../imgs</code> This is significantly safer, but one must still be careful to control the values of $a and $b. Both the exact quoting, and explicitly including "-a" are important.

The brace expansion is present in two basic forms, **string lists** and **ranges**.

It can be switched on and off under runtime by using the ''set'' builtin and the option ''-B'' and ''+B'' or the long option ''braceexpand''. If brace expansion is enabled, the stringlist in ''SHELLOPTS'' contains ''braceexpand''.

===== String lists =====

<code>
{string1,string2,...,stringN}
</code>

Without the optional prefix and suffix strings, the result is just a space-separated list of the given strings:
<code>
$ echo {I,want,my,money,back}
I want my money back
</code>

With prefix or suffix strings, the result is a space-separated list of **all possible combinations** of prefix or suffix specified strings:
<code>
$ echo _{I,want,my,money,back}
_I _want _my _money _back

$ echo {I,want,my,money,back}_
I_ want_ my_ money_ back_

$ echo _{I,want,my,money,back}-
_I- _want- _my- _money- _back-
</code>

The brace expansion is only performed, if the given string list is really a **list of strings**, i.e., if there is a minimum of one "'',''" (comma)! Something like ''{money}'' doesn't expand to something special, it's really only the text "''{money}''".

===== Ranges =====

<code>
{<START>..<END>}
</code>

Brace expansion using ranges is written giving the startpoint and the endpoint of the range. This is a "sequence expression". The sequences can be of two types
  * integers (optionally zero padded, optionally with a given increment)
  * characters

<code>
$ echo {5..12}
5 6 7 8 9 10 11 12

$ echo {c..k}
c d e f g h i j k
</code>

When you mix these both types, brace expansion is **not** performed:
<code>
$ echo {5..k}
{5..k}
</code>

When you zero pad one of the numbers (or both) in a range, then the generated range is zero padded, too:
<code>
$ echo {01..10}
01 02 03 04 05 06 07 08 09 10
</code>
There's a chapter of Bash 4 brace expansion changes at [[#new_in_bash_4.0 | the end of this article]].

Similar to the expansion using stringlists, you can add prefix and suffix strings:
<code>
$ echo 1.{0..9}
1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9

$ echo ---{A..E}---
---A--- ---B--- ---C--- ---D--- ---E---
</code>

===== Combining and nesting =====

When you combine more brace expansions, you effectively use a brace expansion as prefix or suffix for another one. Let's generate all possible combinations of uppercase letters and digits:
<code>
$ echo {A..Z}{0..9}
A0 A1 A2 A3 A4 A5 A6 A7 A8 A9 B0 B1 B2 B3 B4 B5 B6 B7 B8 B9 C0 C1 C2 C3 C4 C5 C6
C7 C8 C9 D0 D1 D2 D3 D4 D5 D6 D7 D8 D9 E0 E1 E2 E3 E4 E5 E6 E7 E8 E9 F0 F1 F2 F3
F4 F5 F6 F7 F8 F9 G0 G1 G2 G3 G4 G5 G6 G7 G8 G9 H0 H1 H2 H3 H4 H5 H6 H7 H8 H9 I0
I1 I2 I3 I4 I5 I6 I7 I8 I9 J0 J1 J2 J3 J4 J5 J6 J7 J8 J9 K0 K1 K2 K3 K4 K5 K6 K7
K8 K9 L0 L1 L2 L3 L4 L5 L6 L7 L8 L9 M0 M1 M2 M3 M4 M5 M6 M7 M8 M9 N0 N1 N2 N3 N4
N5 N6 N7 N8 N9 O0 O1 O2 O3 O4 O5 O6 O7 O8 O9 P0 P1 P2 P3 P4 P5 P6 P7 P8 P9 Q0 Q1
Q2 Q3 Q4 Q5 Q6 Q7 Q8 Q9 R0 R1 R2 R3 R4 R5 R6 R7 R8 R9 S0 S1 S2 S3 S4 S5 S6 S7 S8
S9 T0 T1 T2 T3 T4 T5 T6 T7 T8 T9 U0 U1 U2 U3 U4 U5 U6 U7 U8 U9 V0 V1 V2 V3 V4 V5
V6 V7 V8 V9 W0 W1 W2 W3 W4 W5 W6 W7 W8 W9 X0 X1 X2 X3 X4 X5 X6 X7 X8 X9 Y0 Y1 Y2
Y3 Y4 Y5 Y6 Y7 Y8 Y9 Z0 Z1 Z2 Z3 Z4 Z5 Z6 Z7 Z8 Z9
</code>
Hey.. that **saves you writing** 260 strings!

Brace expansions can be nested, but too much of it usually makes you losing overview a bit ;-)

Here's a sample to generate the alphabet, first the uppercase letters, then the lowercase ones:
<code>
$ echo {{A..Z},{a..z}}
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z a b c d e f g h i j k l m n o p q r s t u v w x y z
</code>

===== Common use and examples =====

==== Massdownload from the Web ====
In this example, ''wget'' is used to download documentation that is split over several numbered webpages.

''wget'' won't see your braces. It will see **6 different URLs** to download.

<code>
wget http://docs.example.com/documentation/slides_part{1,2,3,4,5,6}.html
</code>

Of course it's possible, and even easier, to do that with a sequence:

<code>
wget http://docs.example.com/documentation/slides_part{1..6}.html
</code>



==== Generate a subdirectory structure ====

Your life is hard? Let's ease it a bit - that's what shells are here for.

<code>
mkdir /home/bash/test/{foo,bar,baz,cat,dog}
</code>



==== Generate numbers with a prefix 001 002 ... ====
  * Using a prefix:
<code>
for i in 0{1..9} 10; do printf "%s\n" "$i";done
</code>
If you need to create words with the  number embedded, you can use nested brace:
<code>
printf "%s\n" img{00{1..9},0{10..99},{100..999}}.png
</code>
  * Formatting the numbers with printf:
<code>
echo $(printf "img%02d.png " {1..99})
</code>

See the [[#news_in_bash_4.0 | text below]] for a new Bash 4 method.

==== Repeating arguments or words ====

<code>
somecommand -v -v -v -v -v
</code>

Can be written as
<code>
somecommand -v{,,,,}
</code>
...which is a kind of a hack, but hey, it works.

<div round info>
=== More fun ===
The most optimal possible brace expansion to expand n arguments of course consists of n's prime factors. We can use the "factor" program bundled with GNU coreutils to emit a brace expansion that will expand any number of arguments.

<code>
function braceify {
    [[ $1 == +([[:digit:]]) ]] || return
    typeset -a a
    read -ra a < <(factor "$1")
    eval "echo $(printf '{$(printf ,%%.s {1..%s})}' "${a[@]:1}")"
}

printf 'eval printf "$arg"%s' "$(braceify 1000000)"
</code>

"Braceify" generates the expansion code itself. In this example we inject that output into a template which displays the most terse brace expansion code that would expand ''"$arg"'' 1,000,000 times if evaluated. In this case, the output is:

<code>
eval printf "$arg"{,,}{,,}{,,}{,,}{,,}{,,}{,,,,,}{,,,,,}{,,,,,}{,,,,,}{,,,,,}{,,,,,}
</code>
</div>
===== New in Bash 4.0 =====

==== Zero padded number expansion ====

Prefix either of the numbers in a numeric range with ''0'' to pad the expanded numbers with the correct amount of zeros:
<code>
$ echo {0001..5}
0001 0002 0003 0004 0005
</code>

==== Increment ====
It is now possible to specify an increment using ranges:
<code>
{<START>..<END>..<INCR>}
</code>
''<INCR>'' is numeric, you can use a negative integer but the correct sign is deduced from the order of ''<START>'' and ''<END>'' anyways.
<code>
$ echo {1..10..2}
1 3 5 7 9
$ echo {10..1..2}
10 8 6 4 2
</code>

Interesting feature: The increment specification also works for letter-ranges:
<code>
$ echo {a..z..3}
a d g j m p s v y
</code> 

===== See also =====
  * [[syntax:expansion:intro | Introduction to expansion and substitution]]