Bash brace expansion to remove part of filename

bashbrace-expansionstringsyntax

Is it possible to remove rather than adding substring to a filename using bash brace expansion?

Considering the following scenario, one can add a suffix to a filename by using the below technique:

mv offlineimap.conf{,.minimal}

What it does is renaming offlineimap.conf to offlineimap.conf.minimal which is very handy esp. for making backup files (eg. swith .bak extension).

But is it also possible to subtract substrings from given filenames, like so:

mv offlineimap.conf.minimal{,-minimal}

Here I use - as a hypothetical special character to substract the substring.

I expect the second technique to result in offlineimap.conf removing the .minimal suffix from the name of an existing file.

Best Answer

To use a brace command to remove a suffix, such as .minimal from the file offlineimap.conf.minimal, use:

mv offlineimap.conf{.minimal,}

More on brace expansion

The idea here is that brace expansion creates a series of strings using the comma-separated list of strings between the braces:

$ echo a{b,c}
ab ac

In your first use, the first of the two strings is empty:

$ echo a{,c}
a ac

In the desired solution, we switch it so that the second of the two strings is empty:

$ echo a{b,}
ab a

Or:

$ echo offlineimap.conf{.minimal,}
offlineimap.conf.minimal offlineimap.conf

Related Solutions

Bash Seq – Seq and Brace Expansion Failing

for i in {0..1000000} and for i in $(seq 1000000) both build up a big list and then loop over it. That's inefficient and uses a lot of memory.

Use:

for ((i = 0; i<= 1000000; i++))

instead. Or POSIXly:

i=0; while [ "$i" -le 1000000 ]; do
  ...
  i=$(($i + 1))
done

Or:

seq 1000000 | xargs...

To get a file full of CRLFs:

yes $'\r' | head -n 1000000 >  file

Generally, loops should be avoided when possible in shells.

Bash – brace expansion and * expansion

What happens is that bash first expands *.djvu{,.bk} into *.djvu *.djvu.bk, and then does glob-expansion on those. This would explain what you observe: in your case, *.djvu, matches an existing file, say foo.djvu and expands into that, but *.djvu.bk matches no file, and thus expands as itself, *.djvu.bk.

The order of expansion is specified in the bash documentation:

The order of expansions is: brace expansion, tilde  expansion,  parame‐
ter,  variable  and arithmetic expansion and command substitution (done
in a left-to-right fashion), word splitting, and pathname expansion.

I would suggest rewriting your copy command as:

for f in *.djvu; do cp -- "$f" "$f".bk; done

Or perhaps, to avoid the syntactic overhead of an explicit for loop:

parallel -j1 cp -- {} {}.bk ::: *.djvu

(On second thoughts... that's not really much shorter.)

To answer your sub-question "how could it be expanded", one could use a sub-command (example in a directory containing just foo.djvu and bar.djvu):

$ echo $(echo *.djvu){,.bk}
bar.djvu foo.djvu bar.djvu foo.djvu.bk

But that isn't as safe a solution as the for loop or parallel call above; it will break down on file names containing white space.

Best Answer

More on brace expansion

Related Solutions

Bash Seq – Seq and Brace Expansion Failing

Bash – brace expansion and * expansion

Related Question