Question

Here’s my high level problem.

I want to read a number max_age_in_secs from a config file. This number represents the number of seconds we consider a file is “old”. After we read this config, we want to search all files in a directory data and delete all files modified before max_age_in_secs seconds ago.

This has to be done as a shell script, more specifically korn shell.

What I’m doing right now is to touch a dummy file to be modified max_age_in_secs ago, then use find and its ! -newer option to search for files older than that.

I’m having a problem getting a timestamp to pass to touch. The date command on my unix box doesn’t have the option --date so I can’t use that.

I’m looking at perl -e "blah_blah_blah" but I need to pass the max_age_in_secs variable to the perl command. The only way I know to do that is creating a new .pl file and pass arguments to that file. But ideally I would like to have this functionality in one korn script file. A workaround may be constructing a perl_command variable, and execute its content, but I think that’s not secure and vulnerable to injections.

But I’m relatively new to shell scripting and perl, so any help is appreciated.

Answer 1

There is more than one way to pass an environment variable from a shell to a one-line perl command:

• Directly. Most shells (including korn) interpolate expressions inside double quotes.

perl -e "print $VAR"

so in this case $VAR is coming from the shell. This technique has a lot of shortcomings, namely that with an `-e “double quoted expression” it is a pain to use other Perl variables or use a lot of other useful Perl constructions that might be misconstrued by the shell.

• As a command line argument, accessed in the perl script in @ARGV

perl -e 'print "VAR is $ARGV[0]"' $VAR

This will do if you don’t need to use @ARGV (which are implicitly used if you use the -p or -n switches, or if your program uses the default <> operator).

• Through Perl’s %ENV hash.

perl -e 'print "VAR is $ENV{VAR}"'

This mechanism doesn’t interfere with @ARGV