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Comment #1 posted on 2014-11-27 18:23:23 by Tom Rodman

Thx for covering bash substring expansion

Enjoyed your podcast. Thanks for your work. I'll
have to start using the substring feature.
--

Another example:

Ex
$ forwork=Mustang
$ car=forwork
$ echo ${!car}
Mustang

Ex
$ set -- joy pain bliss; myargc=$#; echo ${!myargc}
bliss
$ set -- joy pain bliss; myargc=$#; echo ${!$#}
bash: ${!$#}: bad substitution

More bash tips at:

https://TRodman.com/blog

Comment #2 posted on 2014-11-29 18:22:36 by Mike Ray

Great stuff

Thanks for a great podcast Dave. Learned some stuff I didn't know.

I particulalry like:

echo ${var:?undefined}

And, something I've now incorporated into some scripts I have for conversion of one audio file type into another:

MP3=${M4A%.m4a}.mp3

Comment #3 posted on 2014-11-29 22:41:45 by Dave Morriss

Thanks Tom

Glad you enjoyed the podcast and found it useful.

I didn't want this episode to go too deep into Bash, so I deliberately drew the line at dealing with indirect references and positional parameters. I was almost ready to cover indirection, but finally decided not to. Perhaps next time!

Your example of 'echo ${!$#}' failing is, I assume, because Bash performs just one scan for parameter substitutions. In this case, even if it performed two passes, this would resolve to 'echo ${!3}' which returns nothing.

I tried this:

$ set -v -- joy pain bliss; myargc=$#; eval echo \${!$#}

which does do two passes. First time the backslash is dropped and $# returns 3 and second time Bash executes 'echo ${!3}' which does nothing. It's not illegal this time, but is counter-intuitive.

This one returns 'bliss':

$ set -v -- joy pain bliss; myargc=$#; ind=myargc; eval echo \${!$ind}

Bash is pretty cool!

Comment #4 posted on 2014-11-29 22:54:04 by Dave Morriss

Thanks Mike

This one was fun to do particularly because it helped to drum this stuff into my head.

I find myself using the suffix removal trick quite often. For example, today I typed the following one-liner to make ImageMagick convert some JPEG files to PNG and reduce them to a more manageable size:

for f in P*.JPG; do convert $f -resize 640 ${f%.JPG}.png; done

Glad you found it useful.

Comment #5 posted on 2014-12-04 10:40:14 by Jon Kulp

Geez just when I think I'm pretty good at something, along comes Dave to show me a whole category of cool bash tricks that I never tried before. Thanks :)

Comment #6 posted on 2014-12-05 21:30:57 by Dave Morriss

Thanks Jon

Glad you liked it. Thanks for the feedback :-)

Comment #7 posted on 2014-12-10 19:32:11 by musicpeace

Thanks Dave! & also for Magnatune

This was a really interesting topic to hear over audio, and your notes are great. Great that you mentioned your past podcast about Magnatune; Looks like a great distribution model for artists (&music fans). I look fwd to working through these, as well as other music apis like Soundcloud, as well as hearing your recent show about podcast/audio. Peace ;

Comment #8 posted on 2014-12-13 22:52:16 by Dave Morriss

Magnatune

Yay musicpeace.

Glad you liked the show. Yes Magnatune is great. John Buckman, the founder, is an impressive guy. The music really suits my tastes and is good value. I have a lifetime membership.

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