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Often, I use parameter expansion on the command-line, such as

cp a.{ext1,ext2}

which is expands to

cp a.ext1 a.ext2

However, in many cases I do not use cp, but another tool, let's call it cat, and I would do some output redirection, like

cat a.ext1 > a.ext2

In my case a can vary, or be quite long, so I would like to be able to use some sort of brace expansion, that at least needs only one occurrence of a in the one-liner. How can I obtain that?

I am specifically using

$ bash --version
GNU bash, version 3.2.51(1)

but answers using other shells are also appreciated.

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up vote 10 down vote accepted

You can type:

$ cat very-long-filename.ext1Ctrl+WCtrl+Y>Ctrl+YBackspace2


$ cat very-long-filename.Ctrl+WCtrl+Yext1>Ctrl+Yext2

To really use brace expansion, note that:

cat a.ext1 > a.ext2

Can also be written:

cat > a.ext2 a.ext1

However you cannot do:

cat > a.ext{2,1}

However, you could do:

eval cat \> a.ext{2,1}
share|improve this answer
Interesting edit! With the cat > a.ext{2,1} I get an ambiguous redirect. Any idea why this is? Does { do anything special? – Bernhard Jul 3 '14 at 10:36
@Bernhard You get that ambiguous redirect in bash any time you pass something that expands to more than one argument to a redirection operator. Same in a='a b'; echo > $a. With zsh, that would do some mult_ios (tee the output to both files) – Stéphane Chazelas Jul 3 '14 at 10:39
Does that imply that bash executes the redirect before the expansion? I would think that it expands first, an then operatres (basically like an alias would), but apparently my view is a bit naive. – Bernhard Jul 3 '14 at 10:44
Funny detail: in zsh, cat > a.ext{2,1} other_file does function. But it outputs the content of other_file to both a.ext1 and a.ext2. I don't know how it manages to duplicate the redirection. – lgeorget Jul 4 '14 at 12:45
@lgeorget, as I said, that's the multios feature whereby zsh implements a tee internally. – Stéphane Chazelas Jul 4 '14 at 12:47

What about:-

f=really-long-filename; cat "$f.ext1" > "$f.ext2"
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That works, but is not really what I am looking for, as I would still have to retype the ${PR} – Bernhard Jul 3 '14 at 8:38

Use a shell function

 xyx () { cat "$1".$2 > "$1".$3; }


xyz fie foo fum

expands to

cat fee.foo > fee.fum
share|improve this answer
Extensions .1 and .2 are typically not universal for me. – Bernhard Jul 3 '14 at 11:54
Modified answer to take parameters. – Lenne Jul 4 '14 at 12:28

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