Use a var with more text only if it exists

command ${MYVAR:+--someoption=$MYVAR}

See "Parameter Expansion" in the bash manpage. They refer to this as "Use Alternate Value", but we're including the var in the at alternative.

Sample Output

$ rsync -n -av ${EXCLUDE_PATERN:+--exclude=$EXCLUDE_PATERN} foo/ bar/
sending incremental file list
created directory bar
./
one.txt
one.xyz
three.txt
three.xyz
two.txt
two.xyz

sent 157 bytes  received 33 bytes  380.00 bytes/sec
total size is 174  speedup is 0.92 (DRY RUN)
$ EXCLUDE_PATERN='*.txt'
$ rsync -n -av ${EXCLUDE_PATERN:+--exclude=$EXCLUDE_PATERN} foo/ bar/
sending incremental file list
created directory bar
./
one.xyz
three.xyz
two.xyz

sent 101 bytes  received 24 bytes  250.00 bytes/sec
total size is 87  speedup is 0.70 (DRY RUN)
$

By: pdxdoughnut

2015-11-04 19:47:24

command substitution variable expansion

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What do you think?

Any thoughts on this command? Does it work on your machine? Can you do the same thing with only 14 characters?

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