Bash Quirkiness
Robert P. J. Day
rpjday at mindspring.com
Mon Jul 26 15:08:22 UTC 2004
On Mon, 26 Jul 2004, David Cary Hart wrote:
> Bash is astonishingly fussy about spacing and punctuation.
>
> Here's a (admittedly very inelegant) snippet of code to automatically
> generate exploit abuse complaints from a cron job.
>
> while read evil freq ; do
> if [ "$freq" -gt 250 ] ; then
> a=`host $evil`
> c=`expr "$a" : '.*\(\..*\.net\)'`
> evilisp=${c/\./abuse\@}
> if [ $evilisp > "0" ] ; then
> . . . . .
>
> Note the use of both ' and ` in the line with the "expr." I find that
> they are not interchangeable. Assigning a variable to an 'expr' will not
> work without the "`". In fact, I cut and pasted it from some web docs.
> I'm not even really sure what character it is.
it's short for "command substitution", which runs the command
contained with the ` quotes and produces what's printed to standard
output. *very* different from the regular single quotes -- you *bet*
they're not interchangeable.
to avoid confusion, you can also identify command substitution with
the syntax $(... command ...), which i personally prefer, since it's
much clearer. that is, you'd be better off writing:
c=$(expr ... ')
at least, in my opinion.
rday
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