Day 11: System Interaction suggested reading: Learning Perl (4th - - PowerPoint PPT Presentation

CS 368 – Intro to Scripting Languages

Day 11: System Interaction

suggested reading: Learning Perl (4th Ed.), Chapter 14: Process Management

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CS 368 – Intro to Scripting Languages

Homework Review

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CS 368 – Intro to Scripting Languages

Problem

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DB App #1 App #2 File

?

CS 368 – Intro to Scripting Languages

Problem

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Perl script DB App #1 App #2 File

CS 368 – Intro to Scripting Languages

Arguments

• $0 : command name
• @ARGV : command-line arguments

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#!/usr/bin/perl use strict; use warnings; foreach my $arg (@ARGV) { # process argument } print "$0: arguments parsed\n";

CS 368 – Intro to Scripting Languages

Arguably Better Arguments

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% do-stuff -aXb --lo --with foo file1 use Getopt::Long; my $lo = 0; my $with = ''; GetOptions('lo' => \$lo, 'with=s' => \$with, # etc.); my $file = $ARGV[0];

CS 368 – Intro to Scripting Languages

Environment

• %ENV : environment variables
• readable and writable

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my @path = split(/:/, $ENV{'PATH'}); my @new = grep(! m{/sbin}, @path); $ENV{PATH} = join(':', @new); delete $ENV{'BLAH'};

CS 368 – Intro to Scripting Languages

Running a Command

system()

• May create a shell to parse command
• Waits for command to finish
• Command inherits environment, etc.

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system("gzip $output_file");

(does a fork & exec, FWIW)

CS 368 – Intro to Scripting Languages

Sneaky system() Subtleties

• To shell or not to shell?

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system('ls'); # no system('ls >> my_file'); # yes system('ls', '>>', 'my_file'); # no

• Children do not affect parent

system('pwd'); system('cd subdirectory'); system('pwd');

CS 368 – Intro to Scripting Languages

Sneaky system() Subtleties II

• Watch out for quoting

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system("echo 'don'\\''t say \"no\"!'");  echo 'don'\''t say "no"!'  don't say "no"!

CS 368 – Intro to Scripting Languages

Return Values

• system() returns exit code << 8
• Exit code of 0 is good in shell,
• But 0 is false in Perl…

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system(…) && die('FAIL!'); # confusing

• r

!system(…) || die('FAIL!'); # may miss !

• So, strive for clarity:

system(…) == 0 or die(); if (system(…) != 0) { die(); }

CS 368 – Intro to Scripting Languages

Errors

• return value ≡ $? ≡ exit code << 8
• $? == -1 means it failed to execute
• $! is the system error message

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if ($? == -1) { print "failed to execute: $!\n"; } elsif ($? & 127) { print "died with signal"; } else { print "exited " . ($? >> 8) . "\n"; }

CS 368 – Intro to Scripting Languages

Getting Output

• Like system() but returns stdout
• $? and $! are set in the same way
• Need stderr, too?

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my $sys_date = `date`; my $sys_date = `date 2>&1`;

• List context: one element per line
• Backticks interpolate!

my @files = `find $directory`;

CS 368 – Intro to Scripting Languages

Miscellaneous

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• Quit script with given exit code

exit 1; die('message');

• Print to standard error

print STDERR 'message';

• Exit nonzero with message

CS 368 – Intro to Scripting Languages

When To Use

• To glue existing systems together
• As a more powerful shell
• NOT to replace Perl functions!

– I.e., `date` is probably a bad idea

• Examples from my work:

– Automate a build – Run configuration scripts during install – Check command output for failure

• Your ideas?

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CS 368 – Intro to Scripting Languages

Security Considerations

• BE CAREFUL!
• Especially with non-literal commands
• use taint; may help (but is hard)
• what could go wrong with this?

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system("mv $foo $bar");

CS 368 – Intro to Scripting Languages

Other Scripting Languages

• In command-line scripts, expect

– Command-line arguments – Environment – Standard in, out, and error – System calls – Exit with status – Windows will be different…

• Embedded scripting (PHP, Lua, JS, …)

– Expect more restrictions

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