Brace Expansion

Remarks#

Create directories to group files by month and year

$ mkdir 20{09..11}-{01..12}

Entering the ls command will show that the following directories were created:

2009-01 2009-04 2009-07 2009-10 2010-01 2010-04 2010-07 2010-10 2011-01 2011-04 2011-07 2011-10
2009-02 2009-05 2009-08 2009-11 2010-02 2010-05 2010-08 2010-11 2011-02 2011-05 2011-08 2011-11
2009-03 2009-06 2009-09 2009-12 2010-03 2010-06 2010-09 2010-12 2011-03 2011-06 2011-09 2011-12

Putting a 0 in front of 9 in the example ensures the numbers are padded with a single 0. You can also pad numbers with multiple zeros, for example:

$ echo {001..10}
001 002 003 004 005 006 007 008 009 010

Create a backup of dotfiles

$ cp .vimrc{,.bak}

This expands into the command cp .vimrc .vimrc.bak.

Modifying filename extension

$ mv filename.{jar,zip}

This expands into mv filename.jar filename.zip .

Use increments

$ echo {0..10..2}
0 2 4 6 8 10

A third parameter to specify an increment, i.e. {start..end..increment}

Using increments is not constrained to just numbers

$ for c in {a..z..5}; do echo -n $c; done
afkpuz

Using brace expansion to create lists

Bash can easily create lists from alphanumeric characters.

# list from a to z    
$ echo {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
    
# reverse from z to a
$ echo {z..a}
z y x w v u t s r q p o n m l k j i h g f e d c b a

# digits
$ echo {1..20}
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
    
# with leading zeros
$ echo {01..20}
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20
    
# reverse digit
$ echo {20..1}
20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
   
# reversed with leading zeros
$ echo {20..01}
20 19 18 17 16 15 14 13 12 11 10 09 08 07 06 05 04 03 02 01

# combining multiple braces
$ echo {a..d}{1..3}
a1 a2 a3 b1 b2 b3 c1 c2 c3 d1 d2 d3

Brace expansion is the very first expansion that takes place, so it cannot be combined with any other expansions.

Only chars and digits can be used.

This won’t work: echo {$(date +$H)..24}

Make Multiple Directories with Sub-Directories

mkdir -p toplevel/sublevel_{01..09}/{child1,child2,child3}

This will create a top level folder called toplevel, nine folders inside of toplevel named sublevel_01, sublevel_02, etc. Then inside of those sublevels: child1, child2, child3 folders, giving you:

toplevel/sublevel_01/child1
toplevel/sublevel_01/child2
toplevel/sublevel_01/child3
toplevel/sublevel_02/child1

and so on. I find this very useful for creating multiple folders and sub folders for my specific purposes, with one bash command. Substitute variables to help automate/parse information given to the script.