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I was trying to concatenate text files in sub-folders and tried:

cat ./{mainfolder1,mainfolder2,mainfolder3}/{subfolder1}/book.txt > out$var

However this did not return anything. So, tried adding a non existing 'subfolder2'

cat ./{mainfolder1,mainfolder2,mainfolder3}/{subfolder1,subfolder2}/book.txt > out$var

And this time it did work out, concatenating the files successfully. Why does this happens?

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    Use echo instead of cat to see what command line you got. (Or use set -x for debugging.)
    – Peter Cordes
    May 20, 2018 at 6:46

3 Answers 3

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By definition, brace expansion in GNU Bash requires either a sequence expression or a series of comma-separated values:

Patterns to be brace expanded take the form of an optional preamble, followed by either a series of comma-separated strings or a sequence expression between a pair of braces, followed by an optional postscript.

You can read the manual for details.

A few simple samples:

echo {subfolder1}
{subfolder1}

echo {subfolder1,subfolder2}
subfolder1 subfolder2

echo subfolder{1}
subfolder{1}

echo subfolder{1..2}
subfolder1 subfolder2
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{subfolder1} evaluates to {subfolder1}, since there are no alternatives. Use subfolder1 instead.

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    Note that it's different from in csh (where brace expansion comes from), tcsh or fish.
    – Stéphane Chazelas
    May 19, 2018 at 9:50
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Braces will only expand if they have coma separated strings, for e.g. {abc,def} or range, for e.g. {a..e} specified between them.

In your case you can just write subfolder1 without enclosing it in braces as there is no need for that

cat ./{mainfolder1,mainfolder2,mainfolder3}/subfolder1/book.txt > out$var
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  • Unfortunately, /path/{a,}/filename expands to the two strings /path/a/filename and /path//filename, which may be unwanted.
    – Kusalananda ♦
    May 19, 2018 at 17:02
  • thanks @Kusalananda for rectifying me, yes bash will provide a warning saing "ambiguous redirect"
    – Neo_Returns
    May 19, 2018 at 17:10
  • No, you get ambiguous redirect if you try to redirect into a file given by an unquoted variable that has no value, e.g. echo 'hello' >$idontexist.
    – Kusalananda ♦
    May 19, 2018 at 17:14
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    ...or if the filename in the redirection gets expanded to multiple words. Like > *.txt with multiple .txt files, or > $file if $file contains whitespace. But of course there's nothing ambiguous in giving cat multiple arguments
    – ilkkachu
    May 20, 2018 at 8:33

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