1. Perl Process Management
Learning Objectives:
To learn the different Perl’s commands for invoking system process
To learn how to perform process management in a Perl program
Running Shell Commands – system
Backquotes
Using Processes as Filehandles

2. Running shell command - system (1)
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"; }

3. Running shell command - system (2)
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 $
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 $

4. Running shell command - system (4)
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) $

5. Running shell command - system (6)
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");

6. 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).

7. 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 $

8. 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/ 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) $

9. Using Processes as Filehandles (input)
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 (<INFILE>) {
($who, $where, $when) =/(\S+)\s+(\S+)\s+(.*)/;
print "$who on $where at $when\n"; }

10. Using Processes as Filehandles (1)
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 file
@letter = <FILE>; # read the contents
close(FILE);
@ List =
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

11. Using Processes as Filehandles (2)
This example prints the letter:
$ cat print1
#!/usr/local/bin/perl5 -w
open(FILE, "letter1"); # open the file
@letter = <FILE>; # read the contents
close(FILE);
@nameList = 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