Bash – Possible to use brace (permutation) and array expansion simultaneously

bashbrace-expansionshell-builtinshell-script

Bash's {} brace expansion syntax allows for creating easy permutations

# echo {b,c,d}{a,e,i,o,u}
ba be bi bo bu ca ce ci co cu da de di do du

However it's not clear to me if/how it's possible to use this along with arrays except in very awkward use of $() echo and eval

Is there a simple way to use arrays with curly brace (permutation) expansion?

For example sake, imagine something like (which does not work of course):

CONS=( b c d )
VOWEL=( a e i o u )

echo {${CONS[@]}}{${VOWEL[@]}}

Best Answer

You might use eval with IFS=,; "${array[*]}" (which joins the values with commas) or just two for loops:

$ CONS=(b c d);VOWEL=(a e i o u)
$ IFS=,;eval echo "{${CONS[*]}}{${VOWEL[*]}}"
ba be bi bo bu ca ce ci co cu da de di do du
$ for c in "${CONS[@]}";do for v in "${VOWEL[@]}";do echo "$c$v";done;done|paste -sd' ' -
ba be bi bo bu ca ce ci co cu da de di do du

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

Related Solutions

Bash Seq – Seq and Brace Expansion Failing

Bash – brace expansion and * expansion

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