Brace expansion

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

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

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

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

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

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

echo {a,b}$PATH

echo a$PATH b$PATH

eval echo {$a..$b}

for i in $(eval echo {$a..$b})

declare -a 'pics=(img{'"$a..$b"'}.png)'; mv "${pics[@]}" ../imgs

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.

{string1,string2,...,stringN}

Without the optional prefix and suffix strings, the result is just a space-separated list of the given strings:

$ echo {I,want,my,money,back}
I want my money back

With prefix or suffix strings, the result is a space-separated list of all possible combinations of prefix or suffix specified strings:

$ 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-

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}".

{<START>..<END>}

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

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

$ echo {c..k}
c d e f g h i j k

When you mix these both types, brace expansion is not performed:

$ echo {5..k}
{5..k}

When you zero pad one of the numbers (or both) in a range, then the generated range is zero padded, too:

$ echo {01..10}
01 02 03 04 05 06 07 08 09 10

Similar to the expansion using stringlists, you can add prefix and suffix strings:

$ 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---

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:

$ 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

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:

$ 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

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.

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

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

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

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

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

• Using a prefix:

for i in 0{1..9} 10; do printf "%s\n" "$i";done

printf "%s\n" img{00{1..9},0{10..99},{100..999}}.png

• Formatting the numbers with printf:

echo $(printf "img%02d.png " {1..99})

See the text below for a new Bash 4 method.

somecommand -v -v -v -v -v

Can be written as

somecommand -v{,,,,}

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)"

eval printf "$arg"{,,}{,,}{,,}{,,}{,,}{,,}{,,,,,}{,,,,,}{,,,,,}{,,,,,}{,,,,,}{,,,,,}

Prefix either of the numbers in a numeric range with 0 to pad the expanded numbers with the correct amount of zeros:

$ echo {0001..5}
0001 0002 0003 0004 0005

It is now possible to specify an increment using ranges:

{<START>..<END>..<INCR>}

$ echo {1..10..2}
1 3 5 7 9
$ echo {10..1..2}
10 8 6 4 2

Interesting feature: The increment specification also works for letter-ranges:

$ echo {a..z..3}
a d g j m p s v y

• Introduction to expansion and substitution