Shell – How Does Curly Brace Expansion Work in the Shell?

brace-expansioncommand lineshell

The command echo {1..3}-{1,2} prints 1-1 1-2 2-1 2-2 3-1 3-2. I understand the way those curly braces can be used. But what actually are they?

Is it the job of sh / bash to parse/expand them and deliver the expanded version to the executed program?

If so, what other tricks can it do and is there a specification?

Also, is there a name for it?

Is ls *.txt handled in a similar way internally?

Is there a way to achieve an n-times repetition of an argument? Like (not working, of course, only a concept): cat test.pdf{*3} ⇒ cat test.pdf test.pdf test.pdf ?

Best Answer

They are called brace expansion.

It is one of several expansions done by bash, zsh and ksh, filename expansion *.txt being another one of them. Brace expansion is not covered by the POSIX standard and is thus not portable. You can read on this in bash manual.

On @Arrow's suggestion: in order to get cat test.pdf test.pdf test.pdf with brace expansion alone, you would have to use this "hack": 

#cat test.pdf test.pdf
cat test.pdf{,}

#cat test.pdf test.pdf test.pdf
cat test.pdf{,,}

#cat test.pdf test.pdf test.pdf test.pdf
cat test.pdf{,,,}

Some common uses:

for index in {1..10}; do
   echo "$index"
done

touch test_file_{a..e}.txt

Or another "hack" to print a string 10 times:

printf -- "mystring\n%0.s" {1..10}

Be aware that brace expansion in bash is done before parameter expansion, therefore a common mistake is: 

num=10
for index in {1..$num}; do
   echo "$index"
done

(the ksh93 shell copes with this though)

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