Brace expansion

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

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

<PREAMBLE>{........}

{........}<POSTSCRIPT>

<PREAMBLE>{........}<POSTSCRIPT>

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

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

then the brace expansion does not expand the variable - this is done in a later step. Brace expansion just makes it being:

echo a$PATH b$PATH

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:

eval echo {$a..$b}

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 SHELLOPTIONS contains braceexpand.

{string1,string2,...,stringN}

Without the optional preamble and postscript 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 preamble or postscript strings, the result is a space-separated list of all possible combinations of preamble, specified strings and postscript:

$ 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's minimum 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 zeropad one of the numbers (or both) in a range, then the generated range is zeropadded, too:

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

There's a chapter of Bash 4 brace expansion changes at the end of this article.

Similar to the expansion using stringlists, you can add preamble and postscript 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 preamble or postscribt 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

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:

$ 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

If you need to create words with the number embedded, you can use nested brace:

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{,,,,}

…which is a kind of a hack, but hey, it works.

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>}

<INCR> is numeric, you can use a negative integer but the correct sign is deduced from the order of <START> and <END> anyways.

$ 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