Perl Process Management Learning Objectives: 1. To learn the different Perl’s commands for invoking system process 2. To learn how to perform process management in a Perl program

COMP111 Lecture 23a / Slide 2 Table of Content  Running Shell Commands - system  Backquotes  Using Processes as Filehandles Perl Process Management

COMP111 Lecture 23a / Slide 3  Perl programs can execute shell commands (Bourne shell) using the system function. system("date");  When the command is finished, the system function returns 0 (not 1 !) if the command completed successfully. if(system("date") != 0){ print "Wah! Date failed!\n"; } Running shell command - system (1)

COMP111 Lecture 23a / Slide 4  Where does the command output go?  Answer: The display screen, unless it is redirected. $ cat system1 #!/usr/local/bin/perl5 -w system("date"); $ system1 Thu Apr 15 14:30:03 HKT 1999 $ Running shell command - system (2)

COMP111 Lecture 23a / Slide 5  How to redirect the I/O?  Answer: Same as the shell $ cat system2 #!/usr/local/bin/perl5 -w if(system("date >now")){ die "cannot create file now"; } $ system2 $ cat now Thu Apr 15 14:30:03 HKT 1999 $ Running shell command - system (3)

COMP111 Lecture 23a / Slide 6  system allows multiple commands, separated by semicolons or newlines.  Processes that end in & are run as background jobs (Perl continues and does not wait for the job to finish). $ cat system3 #!/usr/local/bin/perl5 -w $where = "now"; # Perl variable, not shell variable system "(date; who) >$where &"; $ system3 $ cat now Thu Apr 15 14:58:56 HKT 1999 horner pts/0 Apr 15 14:28 (csz096.cs.ust.hk) $ Running shell command - system (4)

COMP111 Lecture 23a / Slide 7  In the previous example, $where is replaced with its value (by Perl, not the shell).  If you want to reference a shell variable named $where, you need to put a backslash in front of the dollar sign.  The invoked shell command will inherit the current working directory from the Perl program. Running shell command - system (5)

COMP111 Lecture 23a / Slide 8  How would you look for the string “ Bill Gates ” in the file bill ? $ grep "Bill Gates" bill  Answer: system allows the first argument to be the command, and the remaining arguments to be the arguments to the command. system("grep", "Bill Gates", "bill"); Running shell command - system (6)

COMP111 Lecture 23a / Slide 9 Backquotes (1)  How do you assign the output of a command to a variable?  Answer: Use backquotes. $now = `date`;  The value of $now will contain the result of the date commend (including the trailing newline).

COMP111 Lecture 23a / Slide 10 Backquotes (2)  Another example: $ cat back1 #!/usr/local/bin/perl5 -w $where = `pwd`; print "The current directory is: $where"; $ back1 The current directory is /homes/horner $

COMP111 Lecture 23a / Slide 11 Backquotes (3)  If the backquoted command is used in a list context, you get a list of strings, each one being a line (terminated in a newline). $ who horner pts/0 Apr 15 14:28 (csz096.cs.ust.hk) $ cat back2 #!/usr/local/bin/perl5 -w foreach $_ (`who`){ ($who, $where, $when) = /(\S+)\s+(\S+)\s+(.*)/; print "$who on $where at $when\n"; } $ back2 horner on pts/0 at Apr 15 14:28 (csz096.cs.ust.hk) $

COMP111 Lecture 23a / Slide 12  If we use a pipe instead of backquote, the Perl program reads input from the previous pipe stage and is therefore more memory efficient and need not wait for the previous program to finish. $ cat whopipe #!/usr/local/bin/perl5 -w open(INFILE, "who|") || die("Cannot run who…\n"); while ( ) { ($who, $where, $when) =/(\S+)\s+(\S+)\s+(.*)/; print "$who on $where at $when\n"; } Using Processes as Filehandles (input)

COMP111 Lecture 23a / Slide 13  Another way to run a shell command looks similar to filehandles, using open, close, and print.  The following example allows you to customize a letter and automatically it to different recipients: $ cat mail1 #!/usr/local/bin/perl5 -w open(FILE, "letter1");# open the = ;# read the contents = foreach $ open(MAIL, "|mail -s \"hi!\" $ "); #open message print MAIL "Hi $ ,\n"; # add header to message print # add content to message close MAIL; # send message! } $ mail1 Using Processes as Filehandles (1)

COMP111 Lecture 23a / Slide 14  This example prints the letter: $ cat print1 #!/usr/local/bin/perl5 -w open(FILE, "letter1");# open the = ;# read the contents = qw(Andrew Bill Bob); foreach $name open(PRINT, "|lpr -Pcll3"); # open print job print PRINT "Hi $name,\n"; # add header to job print # add content to job close PRINT; # send print job! } $ print1 Using Processes as Filehandles (2)