When do you use brace expansion?

Question

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.

Answer 1

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.com/book/page{1..5}.html

or

for i in {1..100}
do
   #do something 100 times
done

Answer 2

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.

Answer 3

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

Answer 4

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

Answer 5

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