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I understand what brace expansion is, but I don't know how best to use it.

When do you use it?
Please teach me some convenient and remarkable examples if you have your own tip.

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1
  • For the record, {a,b} brace expansion was first introduced in csh in the late 70s and the {1..10} {001..100} {a-z} variant by zsh in the early 90s, and yet more variations in ksh93 in the mid 2000s. Beside the shells you've already tagged, it's also available in fish and yash.
    – Stéphane Chazelas
    Jun 22, 2022 at 19:10

7 Answers 7

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60

Brace expansion is very useful if you have long path names. I use it as a quick way to backup a file:

cp /a/really/long/path/to/some/file.txt{,.bak}

will copy /a/really/long/path/to/some/file.txt to /a/really/long/path/to/some/file.txt.bak

You can also use it in a sequence. I once did so to download lots of pages from the web:

wget http://domain.example/book/page{1..5}.html

or

for i in {1..100}
do
   #do something 100 times
done
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  • 9
    This can be used for argument duplication as well. Say you have some program which requires a '-f' argument for every file to operate on. instead of doing program -f file1 -f file2 -f file3, you can do program "-f file"{1..3}
    – phemmer
    Feb 1, 2011 at 20:42
  • This is a shame. I just discovered it but do the backup thing on extra-long paths for years without knowing rhis. Thanks.
    – smonff
    May 20, 2014 at 12:41
  • 2
    @Patrick: That fails because program gets three words: "-f file1" "-f file2" "-f file3", instead of 6: "-f" "file1" "-f" "file2" "-f" "file3".
    – musiphil
    May 18, 2015 at 18:52
  • 1
    @dogbane: Using expansion for use in a for loop is a bit pointless, since you can write for ((i=1; i<=100; i++)) and it is more efficient.
    – musiphil
    May 18, 2015 at 18:54
37
+50

Brace expansion comes very handy when creating large directory structures:

mkdir -p dir1/{subdir1,subdir2}/{subsubdir1,subsubdir2}

This will give you

find dir1 -type d
dir1
dir1/subdir1
dir1/subdir1/subsubdir1
dir1/subdir1/subsubdir2
dir1/subdir2
dir1/subdir2/subsubdir1
dir1/subdir2/subsubdir2

You could even go one step further and put brace expansion into brace expansion:

mkdir -p dir1/{subdir{1,2}}/{subsubdir{1,2}}

This will give you the same directory structure as the example above.

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  • 3
    Nesting is nice, but your nesting example (mkdir -p dir1/{subdir{1,2}}/{subsubdir{1,2}}) doesn't actually serve any purpose. You could have just done this: mkdir -p dir1/subdir{1,2}/subsubdir{1,2}.
    – iconoclast
    Jul 14, 2011 at 18:40
  • @iconoclast, it does serve a purpose if you interpret subdir1 and subdir2 in the non-literal sense. Replace subdir1 with cat and subdir2 with dog for example.
    – ephsmith
    Jul 26, 2012 at 10:55
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    @ephsmith: if you interpret "subdir1" as standing in for "cat" and "subdir2" for "dog", then the nesting example fails, since it uses subdir{1,2}, and there is no string that you can replace "subdir" with that will make subdir{1,2} return {cat,dog}.
    – iconoclast
    Jul 26, 2012 at 21:04
23

I use it when I want to reduce typing:

geany /path/to/file1 /path/to/file2
# versus
geany /path/to/file{1,2}

Another example:

wajig install libpam0g-dev libiw-dev libdb-dev
# versus
wajig install lib{pam0g,iw,db}-dev
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13

I use it to compare actual test output to desired test output during development. If test #41 fails, it's easy to see what the difference between the test output (in file tests.output/041) and the desired output (in file tests.out/041):

$ diff tests.{out,output}/041
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12

Some frequent cases for me are:

For renaming:

mv myText.{txt,tex}

or

mv myText.tex{,.old}

or

cp myText.tex{,.backup}

(Although it's less messy to use version control for the last 2 tasks.)

For comparing (already mentioned):

diff path{1,2}/a.txt
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4

There are several great answers here, but none of them mention when not to use brace expansion. Like the other answerers, I use brace expansion on the command line quite a bit. I also use it in my aliases and functions since I can expect a smart shell.

I do not use it in my shell scripts (unless there's some other reason the scripts should be bash or zsh, though in those cases, it's best to upgrade to a "real" scripting language like perl or python). Brace expansion is not portable since it is not in the POSIX shell standard. Even if it works in your /bin/sh-shebanged shell scripts, it will not work on systems with more stripped /bin/sh shells (e.g. dash).

In the case of a difference of a single digit, you don't need brace expansion; a character class will suffice:

Bashism:

diff file{1,2}

Portable:

diff file[12]
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  • 6
    A difference is that file{1,2} will always expand to file1 file2, while file[12] expands only to existing filenames: i.e. if file1 exists but file2 doesn't, file[12] expands to file1 only. The [] expansion is really a restricted version of ? expansion (and they are called "pathname expansions").
    – musiphil
    May 18, 2015 at 18:57
  • Correct, brace expansion is not shell globbing (a.k.a. pathname expansion). That's a good call-out that hasn't really been mentioned yet: brace expansion is better at noting unexpectedly absent paths. You can also glob inside brace expansion.
    – Adam Katz
    May 18, 2015 at 21:28
2

After splitting a very big file with

split -b 100m hugeFile.zip part.

let's suppose it splitted into 262 pieces, in other words part.aa,part.ab,part.ac ... part.kb

Your can join those pieces again by nesting brace expansions, like this

cat part.{{a..j}{a..z},k{a..b}} > hugeFile.zip
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