I didn't really want to write three articles about bash in a row, but after my last article about Bash prompts Ralph Aichinger emailed me asking about a feature he had in zsh, where his xterms show him the current process and whether it was possible to do that in Bash. Never one to refuse a challenge, I had a go and my latest article is the result.

I managed to add two more sections to my article on writing robust shell scripts including using trap and making more atomic changes. Had some useful feedback including making the fact that a few small tweaks made it apply to more than just bash. Following from that I've added an article about changing your bash prompt and how mine has been built up over the years to something useful to me. Hopefully it'll give other people some ideas.

I've just finished writing an article on tips for making your shell scripts more robust. Comments welcome.

Yesterday I posted about some bad shell code I had found and posted an improved version. Part of the reason for posting it was that I hoping someone could point out any errors in the version I posted. Fortunately Neil Moore emailed me some improvements.

1. If the script returns more than one line they will be removed by the $(...) expansion when it is split into words. The solution there is to surround it in double quotes.
2. The next problem Neil pointed out was that $@ should be surrounded by quotes in pretty much every case, otherwise parameters with spaces in will get split into separate parameters.
3. The final problem is that if the script includes a return statement, it will stop the inner most function or sourced script, but not during eval.The solution is to enclose it in a function:

   
   dummy() { eval "$(perl "$CONF_DIR/project.pl" "$@")"; } dummy "$@"
   

Since making the post, I discovered that Solaris' /bin/sh doesn't like $(...), so it's probably better to use backticks instead if you want to be portable. As I know the output from the script I'm not worried about return statements, so I've ended up with:

eval "`perl "$CONF_DIR/project.pl" "$@"`"

I'm always amazed at the number of bad shell scripts I keep coming across. Take this snippet for example:

TEMP1=`mktemp /tmp/tempenv.XXXXXX` || exit 1
perl $CONF_DIR/project.pl $@ > $TEMP1
if [ $? != 0 ]; then
  return
fi

. $TEMP1
rm -f $TEMP1

There are several things wrong with this. First it uses a temporay file. Secondly it uses more processes than are required and thirdly it doesn't clean up after itself properly. If perl failed, the temp file would still be created, but not deleted. The last problem can be solved with some suitable uses of trap:

TEMP1=`mktemp /tmp/tempenv.XXXXXX` || exit 1
trap "rm -f $TEMP" INT TERM EXIT
perl $CONF_DIR/project.pl $@ > $TEMP1
if [ $? != 0 ]; then
  return
fi

. $TEMP1
rm -f $TEMP1
trap - INT TERM EXIT

Of course this can all be replaced with a single line:

eval $(perl $CONF_DIR/project.pl $@)