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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?

Charles Duffy
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CDPF
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3 Answers3

22

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
Community
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Arthur Hess
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21

{subfolder1} evaluates to {subfolder1}, since there are no alternatives. Use subfolder1 instead.

David Foerster
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Ignacio Vazquez-Abrams
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1

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$var
Neo_Returns
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  • Unfortunately, `/path/{a,}/filename` expands to the _two_ strings `/path/a/filename` and `/path//filename`, which may be unwanted. – Kusalananda May 19 '18 at 17:02
  • thanks @Kusalananda for rectifying me, yes bash will provide a warning saing "ambiguous redirect" – Neo_Returns May 19 '18 at 17:10
  • No, you get `ambiguous redirect` if you try to redirect into a file given by an unquoted variable that has no value, e.g. `echo 'hello' >$idontexist`. – Kusalananda May 19 '18 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 '18 at 08:33