Note by the way that the -e flag
Friday, October 27th, 2006Note by the way that the -e flag has to be the last one before code; you can’t type perlew -p-n: Execute codefor every line from the input, and either print $_ automatically when done (-p) or not (-n). Say you want to find out the ASCII codes generated by certain keys on your keyboard; you could type % perl -pe ’s/(.)/” “.ord $1/ge’ and then hit a key followed by a newline to see its ASCII value. -l (the letter ‘ell’, not the number one): Append an automatic newline to printstatements. When combined with -nor -p, also automatically chomp the newline character off the input. Let’s say you want to find which users listed in a Unix /etc/passwdfile have directories under /home: % perl -nle ’s/:.*//; print if -d “/home/$_”‘ /etc/passwd This can most usefully be used as a first stage in a pipeline of commands that then go on to do other things with its results. Or you might want a Perl program as the final stage of a pipeline that periodically reports how many lines it has read: % … | perl -nle ‘BEGIN{ $SIG{ALRM} = sub{ print $.; alarm 10 }; alarm 10 }’ In this case, nothing at all is being executed in the loop for each line of input! The -l flag is particularly useful in one-liners or in short test programs in which you have only a handful of printstatements and want to save the trouble of typing nin all of them. However, don’t leave it in production code; setting a global flag like this affects the behavior of all print statements and will produce unpleasant results from using modules that don’t expect it (such as CGI.pmand Benchmark). -Mmodule: Perform a usemodule before executing any code. You can use this to impose strictness on your one-liners, and also as a quick test to see if a module you want is installed: % perl -MTime::HiRes -e 0 This prints nothing if the Time::HiResmodule is installed but complains if it isn’t. -c: This flag gives you the ability to check the syntax of a Perl program without actually running it. Because so much error checking is deferred until run-time in Perl,
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