lamby

Scrap your boilerplate; POSIX shell edition

I don't remember where I saw this first - probably in the Debian Installer code. Anyway, given an n-tuple it can be somewhat unwieldy to unpack it:

VAR="Aho Sethi Ullman"
ONE="$(echo ${VAR} | cut -d' ' -f1)"
TWO="$(echo ${VAR} | cut -d' ' -f2)"
THREE="$(echo ${VAR} | cut -d' ' -f3)"

The trick being presented is to generate the boilerplate code and then eval it:

VAR="Aho Sethi Ullman"
eval $(echo ${VAR} | awk '{ print "ONE=" $1 " TWO=" $2 " THREE=" $3 }')

Here's a more concrete example, with apologies to Python:

for PAIR in \
           "one    Aho" \
           "two    Sethi" \
           "three  Ullman"
do
    eval $(echo ${PAIR} | awk '{ print "k=" $1 " v=" $2 }')

    # .. do something with $k and $v
done

§

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Five comments

1. nice trick, but it fails if the input data contains the same quotes you use to try to protect it from the eval

   (quoting: number one reason to hate shell code)

   Joey Hess

   It also fails at spaces and literal $'s in the source string but I'm trying to avoid the details. Did I read somewhere you wrote OOP extensions for sh?

   — lamby

2. Better yet:

   #!/bin/bash
   ...
   VAR="Aho Sethi Ullman"
   ...
   a=($VAR)
   # do stuff with ${a[0]}, ${a[1]}, ${a[2]}
   

   Anonymous

   I hope it's obvious to everyone else why this "improvement" is not valid here.

   — lamby

3. In most of these cases, you ought to let the shell do the splitting, e.g.

   set -- Aho Sethi Ullman
   echo $3 $2 $1

   or

   while read k v; do something $k $v; done <<_eof
   red green
   blue yellow
   _eof

   madduck

   Using `set` is nice (especially as you can change what you are splitting on with by altering IFS) but it unconditionally overwrites $1/$2, etc which makes me uncomfortable. And is multi-variable `read` actually POSIX?

   — lamby

4. Here's something of the type which I did the other day:

   #!/bin/bash
   pos -l8 $* | { read LAT LONG
   echo Latitude = $LAT
   echo Longitude = $LONG
   # gnome-open "http://maps.google.com/?ie=UTF8&ll=${LAT},${LONG}&spn=0.003117,0.009656&z=17"
   gnome-open "http://www.openstreetmap.org/?lat=${LAT}&lon=${LONG}&zoom=18&layers=B000FTF"
   }

   ("pos" is my own little utility which converts various flavours of latitude/longitude or Ordnance Survey grid reference to various other flavours of the same.)

   The "scope" of the result can be regarded as a PITA or quite elegant depending...

   Ed Davies

5. A bit off-topic here, but still... Yesterday I had to fiddle a bit with bash quoting and evaluation, as I am working on a specific-purpose system installer - The only user interaction I have is via this loop:

   for conf in HD CDR SWAP_SIZE CONF_URL
   do
   echo -n "$conf: (default = ${!conf}): "
   read tempvar
   if [ ! -z "$tempvar" ]
   then
   eval ${conf}=\$tempvar
   fi

   done

   I had to squeeze my brain (and bash's manpage) for the indirect expansion (${!conf}) and for the proper way for the assignment... It is still breakable, but tolerably so at least.

   Gunnar

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