Shell Programming and Scripting

Hi Robin,
In POSIX-conforming shells (or any shell based on Bourne shell syntax that I know of), field splitting and pathname expansions do not occur when assigning a string to a variable. So, in your example, the assignment:

Code:

filename=INPUT_GIANT_*.csv

shouldn't cause any problem. But if the file's name had been:

Code:

INPUT_GIANT_123;rm -rf * .csv

then in some contexts, expanding $filename unquoted could result in attempting to remove all files in the file hierarchy rooted in the current directory (except hidden files in the current directory) and then attempting to remove the hidden file named .csv from the current directory and, if it was a directory, all files in the file hierarchy rooted in that directory.

Thanks Don,

Maybe I'm being paranoid, but I feel sure there are things going to bite me

I just know that I've been caught out before when trusting filenames on disk, standard messages in log files etc. so I'm just always careful when reading input to try not to assume that it's all good. If this discussion makes people consider the bad things that can happen, then I'm all for it. The worst example I found was the evil eval being used to execute files, something like:

Code:

#
#
# Setup and code for creating a set of 'build-scripts' that are then
# run in parallel for no good reason that I could ever work out
#
# then it went:.....

for file in *.build-script
do
   eval $file > $file-out &
done
wait
exit 0

How may ways was this so wrong? Well, it's all gone now but it highlights what a mess we can make if we're not careful

Having inherited it, it ran in the overnight batch for a few months silently doing the job, but it went VERY badly wrong and it was effectively DR time as we lost /etc making login a little difficult .......

Hi Robin,
You're not being paranoid. Assigning a pathname matching pattern to a variable won't expand the pattern during the assignment (so that isn't inherently dangerous). But, expanding that variable unquoted is a completely different matter. And expanding it with eval (quoted or unquoted) gives you MUCH more to be concerned about.