Perl String Interpolation in Bash Command

Question

I'm trying to use GNU Date to get the seconds between two dates. The reason I'm using GNU Date is for performance (in testing was 10x faster than Perl) for this purpose. However, one of my arguments is a perl variable. Like this:

my $b_row="2012-01-05 20:20:22";
my $exec =qx'CUR_DATE=`echo $(date +"%F %T")` ; echo $(($(date -d  "$CUR_DATE" +%s)-$(date -d  "$b_row" +%s)))';

The problem is that b_row is not being expanded. I've tried a couple different solutions (IPC::System::Simple) being one, tried adjusting the backticks etc. No success, any ideas how to do this appropriately? The main thing is I need to capture the output from the bash command.

How were you doing the calculation in Perl that wound up 10x slower than starting 4 processes? I suspect that it can be done more efficiently. — cjm
– cjm, Commented Jan 6, 2012 at 2:14

Sean · Accepted Answer · 2012-01-05 22:17:41Z

1

Make it easier on yourself and do the minimum amount of work in the shell. This works for me:

my $b_row = '2012-01-05 20:20:22';

my $diff = qx(date -d "\$(date +'%F %T')" +%s) -
           qx(date -d            "$b_row" +%s);

Just be absolutely sure $b_row doesn't have any shell metacharacters in it.

answered Jan 5, 2012 at 22:17

Sean

30k5 gold badges85 silver badges111 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

user3458 · Accepted Answer · 2012-01-05 21:22:53Z

0

That's because you use ' :

           Using single-quote as a delimiter protects the command from
           Perl's double-quote interpolation, passing it on to the shell
           instead:

               $perl_info  = qx(ps $$);            # that's Perl's $$
               $shell_info = qx'ps $$';            # that's the new shell's $$

answered Jan 5, 2012 at 21:22

user3458

3 Comments

Dangeruss Over a year ago

Arkadiy, thanks for your response. I tried that earlier, I get this error: sh: -c: line 0: syntax error near unexpected token (' sh: -c: line 0: CUR_DATE=echo 0 0date +"%F %T") ; echo 0 0(0 0date -d "" +%s)-0 0date -d "2012-01-05 20:20:22" +%s)))'

Dangeruss Over a year ago

It seems to have a problem dealing with the (()) in the date calculation.

user3458 Over a year ago

It probably interpreting %s as a hash. Use \% instead.

mob · Accepted Answer · 2012-01-05 22:36:41Z

qx has the feature of letting you choose a convenient delimiter, including the option of whether to interpolate the string or not (by choosing ' as the delimiter). For this use case, sometimes you want interpolation and sometimes you don't, so qx (and backticks) may not be the right tool for the job.

readpipe is probably a better tool. Like the system EXPR command, it takes an arbitrary scalar as input, and you have all of Perl's tools at your disposal to construct that scalar. One way to do it is:

my $exec = readpipe 
    'CUR_DATE=`echo $(date +"%F %T")` ;'   # interp not desired
    . ' echo $(($(date -d  "$CUR_DATE" +%s)-$(date -d  "'
    . qq/"$b_row"/                         # now interp is desired
    . ' +%s)))';                           # interp not desired again

Collectives™ on Stack Overflow

Perl String Interpolation in Bash Command

3 Answers 3

Comments

3 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related