Perl Process Management

1
Software Tools

• Perl Process Management

2
system

• 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
system

• 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 143003 HKT 1999

4
system

• How to redirect the I/O?
• Answer Same as the shell
• cat system2
• !/usr/local/bin/perl5 -w
• if(system("date gtnow"))
• die "cannot create file now"
• system2
• cat now
• Thu Apr 15 143003 HKT 1999

5
system

• 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) gtwhere "
• system3
• cat now
• Thu Apr 15 145856 HKT 1999
• horner pts/0 Apr 15 1428 (csz096.cs.ust.hk)

6
system

• 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.

7
system

• 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")

8
Backquotes

• 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).

9
Backquotes

• Another example
• cat back1
• !/usr/local/bin/perl5 -w
• where pwd
• print "The current directory is where"
• back1
• The current directory is /homes/horner

10
Backquotes

• 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 1428
  (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 1428
  (csz096.cs.ust.hk)

11
Using Processes as Filehandles

• 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 email it to different
  recipients
• cat mail1
• !/usr/local/bin/perl5 -w
• open(FILE, "letter1") open the file
• _at_letter ltFILEgt read the contents into
  _at_letter
• close(FILE)
• _at_emailList qw(horner_at_cs.ust.hk
  horner_at_ust.hk)
• foreach email (_at_emailList)
• open(MAIL, "mail -s \"hi!\" email") open
  message
• print MAIL "Hi email,\n" add header to
  message
• print MAIL _at_letter add content to message
• close MAIL send message!
• mail1

12
Using Processes as Filehandles

• This example prints the letter
• cat print1
• !/usr/local/bin/perl5 -w
• open(FILE, "letter1") open the file
• _at_letter ltFILEgt read the contents into
  _at_letter
• close(FILE)
• _at_nameList qw(Andrew Bill Bob)
• foreach name (_at_nameList)
• open(PRINT, "lpr Pcll2a") open print job
• print PRINT "Hi name,\n" add header to
  job
• print PRINT _at_letter add content to job
• close PRINT send print job!
• print1