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.
Tag Archives: shell
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.
-
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. -
The next problem Neil pointed out was that $@ should be
surrounded by quotes in pretty much every case, otherwise parameters
with spaces in will get split into separate parameters. -
The final problem is that if the script includes a return statement, it
will stop the inner most function or sourced script, but not during
eval.The solution is to enclose it in a function:dummy() { eval "$(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 $@)