Bash – Brace expansion with elements of an array

bashbrace-expansion

In Bash we can already do this:

echo foo.{a,b,c}
# == foo.a foo.b foo.c

How do we get roughly:

arr=(a b c)
echo foo.{${arr[@]}}
# == foo.a foo.b foo.c

Best Answer

You can use parameter expansion

$ arr=(a b c)

$ echo "${arr[@]/#/foo.}"
foo.a foo.b foo.c

Related Solutions

Bash Scripting – How to Create a Sequence with Leading Zeroes Using Brace Expansion

Prefix the first number with a 0 to force each term to have the same width.

$ echo {08..10}
08 09 10

From the bash man page section on Brace Expansion:

Supplied integers may be prefixed with 0 to force each term to have the same width. When either x or y begins with a zero, the shell attempts to force all generated terms to contain the same number of digits, zero-padding where necessary.

Also note that you can use seq with the -w option to equalize width by padding with leading zeroes:

$ seq -w 8 10
08
09
10

$ seq -s " " -w 8 10
08 09 10

If you want more control, you can even specify a printf style format:

$ seq -s " " -f %02g 8 10
08 09 10

Bash combining wildcard expansion with brace expansion

The shell expands * only if un-quoted, any quoting stops expansion by the shell.

Also, a brace expansion needs to be unquoted to be expanded by the shell.

This work (lets use echo to see what the shell does):

$ echo *.{ext1,ext2}
a.ext1 b.ext1 a.ext2 b.ext2

Even if there are files with some other names:

$ touch {a,b}.{ext1,ext2} {c,d}.{ext3,ext4} none
ls
a.ext1  a.ext2  b.ext1  b.ext2  c.ext3  c.ext4  d.ext3  d.ext4  none

$ echo *.{ext1,ext2}
a.ext1 b.ext1 a.ext2 b.ext2

Why that works?

It is important that we understand why that works. It is because of the order of expansion. First the "Brace expansion" and later (the last one) "Pathname Expansion" (a.k.a glob-expansion).

Brace --> Parameter (variable) --> Pathname

We can turn off "Pathname expansion" for a moment:

$ set -f
$ echo *.{ext1,ext2}
*.ext1 *.ext2

The "Pathname Expansion" receives two arguments: *.ext1 and *.ext2.

$ set +f
$ echo *.{ext1,ext2}
a.ext1 b.ext1 a.ext2 b.ext2

The problem is that we can not use a variable for the brace expansion.
It has been explained many times before for using a variable inside a "Brace Expansion"

To expand a "Brace Expansion" that is the result of a "Variable Expansion", you need to re-submit the command line to the shell with eval.

$ list={ext1,ext2}
$ eval echo '*.'"$list"

Brace --> Variable --> Glob || --> Brace --> Variable --> Glob
........ quoted here -->^^^^^^ || eval ^^^^^^^^^^^^^^^^^^^^^^^^^

Values of the file names bring no execution problem for eval:

$ touch 'a;date;.ext1'
eval echo '*.'"$list"
a;date;.ext1 a.ext1 b.ext1 a.ext2 b.ext2

But the value of $list could be unsafe. However, the value of $list is set by the script writer. The script writer is in control of eval: Just not use externally set values for $list. Try This:

#!/bin/bash
touch {a,b,c}.ext{1,2}
list=ext{1,2}
eval ls -l -- '*.'"$list"

A better alternative.

An alternative (without eval) is to use Bash "Extended Patterns":

#!/bin/bash
shopt -s extglob
list='@(ext1|ext2)'
ls -- *.$list

Note: Please be aware that both solutions (eval and patterns) (as written) are safe for filenames with spaces or new lines. But will fail for a $list with spaces, because $list is unquoted or the eval removes the quotes.