17 - Command-line Options

Perl has a wide range of command-line options or switches that you can use. The options are also called switches because they can turn on or turn off different behaviors. A thorough knowledge of the command line switches will enable you to create short one-time programs to perform odd little tasks. For example, the -e option lets you specify a line of code directly on the command-line instead of creating a script file. You use the -l option to change the line endings in a text file.

How Are the Options Specified?

 perl -I/usr/~john/include script.pl

perl -cI/usr/~john/include script.pl
perl -c -I/usr/~john/include script.pl

#!/bin/perl -w

Caution

Some UNIX systems will only read the first 32 characters of the #! line. So try to have your options either end before the 32nd position or start after the 32nd position. Placing the options after the 32nd position will help to make your scripts more portable because you will be bypassing one of the inconsistencies of UNIX.

What Are the Options?

As you can see, Perl has quite a few command-line options. Most of them are designed so that you can do useful things without needing to create a text file to hold the script. If you are a system administrator then these options will make you more productive. You'll be able to manipulate files and data quickly and accurately. If you're looking to create applications or more complicated programs, you won't need these options - except for -w and -d.

The rest of the chapter is devoted to demonstrating the -0, -n, -p, -i, and -s options.

Example: Using the -0 Option

perl -e "print ord('-');"

Next, you'll need an input file to practice with. Listing 17.1 shows a sample input file.

Listing 17.1-17LST01.DAT - Test Input File for the -0 Option

Veterinarian-Orthopedist-Dentist-

Listing 17.2 holds a program that reads the above data file using the diamond operators. The program will use the dash character as an end-of-line indicator.

Pseudocode

Set the record separator to be a dash using the #! switch setting method. Open a file for input. Read all of the records into the @lines array. One element in @lines will be one record. Close the file. Iterate over the @lines array and print each element.

Listing 17.2-17LST02.PL - Using the -0 Option to Change the Record Separator

#!perl -0055 open(FILE, "<17LST01.DAT"); @lines = <FILE>; close(FILE); foreach (@lines) { print("$_\n"); }

Tip

Instead of using the command-line option, you could also say $/ = "-";. Using the command-line is a better option if the line ending changes from input file to input file.

This program will display:

Veterinarian-
Orthopedist-
Dentist-

Example: Using the -n and -p Options

The -n option causes Perl to execute your script inside the following loop:

while (<>) {
    # your script
}

while (<>) {
    # your script
} continue {
    print;
}

while (<>) {
    @F = split(/ /);
    # your script
}

while (<>) {
    @F = split(/|+/);
    # your script
}

BEGIN {
    # initialization section
    $count = 0;
}

while (<>) {
    # your script
}

END {
    # cleanup section
    print("The count was $count.\n");
}

Next, you'll see some examples of these options in action. Let's start with a command-line that simply displays each line of the input file - like the "type" command in DOS and UNIX.

The following examples use a data file called data.dat with the following lines:

  David Veterinarian
  John Orthopedist
  Jeff Dentist
  

perl -p -e "1;" data.dat

while (<>) {
    1;
} continue {
    print;
}

And will display:

David Veterinarian
John Orthopedist
Jeff Dentist

perl -p -e "s/\s*(\w+).*/$1/;" test.dat

while (<>) {
    s/\s*(\w+).*/$1/;
} continue {
    print;
}

David
John
Jeff

perl -p -e "s/\s*.+\s(.+)\s*/$1\n/;" test.dat

Veterinarian
Orthopedist
Dentist

perl -p -a -e "$_ = \"$F[1]\n\";" test.dat

 while (<>) {
    @F = split(/ /);
    $_ = "$F[1]\n";
} continue {
    print;
}

Notice you need to escape the double-quotes in the above command-line. If you don't do this you get an error message.

Example: Using the -i Option

One of the more popular uses for the -i option is to change sequences of characters. This kind of change normally requires 10 or more lines of code. However, using command line options you can do it like this:

perl -p -i.bak -e "s/harry/tom/g;" test.dat

Examples: Using the -s Option

Listing 17.3-17LST03.PL - Checking for the useTR Switch

if ($useTR) { # do TR processing. print "useTR=$useTR\n"; }

You might execute this program using this following command line:

perl -s -w 17lst03.pl -useTR

useTR=1

Summary

The switches can be specified on the command line or using the #! line inside your script. If you use the #! line, try to place the options after the 32^nd position to avoid inconsistent handling by different versions of UNIX.

The -n option is used to place your script inside of an input loop. The -p option uses the same loop, but also prints the $_ variable after each pass through the loop. The -a and -F options are used when you want the input lines to be split into the @F array.

Another very useful option is -i, which lets you edit files in-place. This option is good when you are doing a lot of text file manipulation.

The next chapter, "Using Internet Protocols," introduces you to some of the different standards used on the Internet. These standards let you perform activities like read mail, send mail, and transfer files.

Review Questions

1. What is a command line option?

2. What are the two places that the switches can be specified?

3. What switch should always be used?

4. Which switch lets you read records that end with the ~ character instead of the newline?

5. What two options can be used with the -n option?

6. How can you execute a script that someone sent you via E-mail?

7. What happens if you specify both the -v and the -c options?

Review Exercises

1. Use the -v option to see the patchlevel of your version of Perl.

2. Use the chomp or chop function to remove the dash from the end of the records printed by the program listing 17.2.

3. Write a program that uses the -p option to display the third column.

4. Modify the program written in exercise 3 to use a BEGIN block to ask the user which column to display?

5. Create a sample E-mail message that contains a Perl script. Use the -x option to execute it.

6. Modify the E-mail message written for exercise 5 to display any text that appears after the __END__ token. Hint: Use the DATA file handle.