Viewing the Perl code during script execution

You can list any part of your program from within the Perl debugger. The following sections describe debugging commands that perform the display operations.

The l command

The simplest way to list part of your program is with the l command, which lists the next few statements in your program:

  DB<1> l
3:      $dircount = 0;
4:      $curdir = "";
5:      while (1) {
6:              # if we don't have a current directory, get one
7:              if ($curdir eq "") {
8:                      print ("Enter directory to list:\n");
9:                      $curdir = <STDIN>;
10:                     $curdir =~ s/^\s+|\s+$//g;
11:                     $curdir = &followlink($curdir);
12:                     &readsubdirs($curdir);

The l command lists a window of executable statements, which is a group of statements small enough to be easily displayed on your screen. A window usually consists of about ten statements. The line number of each statement is displayed at the beginning of its line.

Entering the l command again displays the window of statements immediately following the last displayed line, which in this example is line 12:

  DB<1> l
13:             }
14:             $curdir = &menudir($curdir);
15:     }
16:
17:
18:     # Find all subdirectories of the given directory,
19:     # and store them in an associative array.
20:     #
21:     # The associative array subscripts and values are:
22:     # <directory name>:       1

You can specify the lines displayed by the l command. If you specify a single line number with the l command, it displays that line:

  DB<1> l 10
10:                      $curdir =~ s/^\s+|\s+$//g;

To display a range of lines, specify the first and last line number, and separate them with a hyphen:

  DB<1> l 10-15
10:                      $curdir =~ s/^\s+|\s+$//g;
11:                      $curdir = &followlink($curdir);
12:                      &readsubdirs($curdir);
13:             }
14:             $curdir = &menudir($curdir);
15:     }

To display a specified number of lines starting at a certain point, supply the starting line number, a + character, and a number of additional lines to display:

  DB<1> l 10+5
10:                      $curdir =~ s/^\s+|\s+$//g;
11:                      $curdir = &followlink($curdir);
12:                      &readsubdirs($curdir);
13:             }
14:             $curdir = &menudir($curdir);
15:     }

You also can use the l command to specify a subroutine to display. To do this, provide the name of the subroutine to display:

  DB<1> l readsubdirs
26:     sub readsubdirs {
27:             local ($dirname) = @_;
28:             local ($dirvar, $subdircount, $name, $index);
29:
30:             # open the current directory;
31:             # $dircount ensures that each file variable is unique
32:             $dirvar = "DIR" . ++$dircount;
33:             if (!opendir ($dirvar, $dirname)) {
34:                     warn ("Can't open $dirname\n");
35:                     return;

This command lists the statements in the subroutine. If the subroutine is too large to fit in a single window, only the first few statements are listed; you can list subsequent statements by entering l with no arguments.

The - Command

You can display the lines immediately preceding the last displayed line by entering the - command. For example, the following - command lists the window of lines immediately preceding the subroutine readsubdirs.

  DB<1> -
16:
17:
18:     # Find all subdirectories of the given directory,
19:     # and store them in an associative array.
20:     #
21:     # The associative array subscripts and values are:
22:     # <directory name>:       1
23:     #       (indicates that directory has been read)
24:     # <directory name>.<num>  the <num>th subdirectory
25:

Subsequent - commands go back further in the file.

The w Command

To list a window of lines containing a specified line, use the w command, and specify the number of the line to be included:

  DB<1> w 7
4:      $curdir = "";
5:      while (1) {
6:              # if we don't have a current directory, get one
7:              if ($curdir eq "") {
8:                      print ("Enter directory to list:\n");
9:                      $curdir = <STDIN>;
10:                     $curdir =~ s/^\s+|\s+$//g;
11:                     $curdir = &followlink($curdir);
12:                     &readsubdirs($curdir);
13:             }

The w command displays the three lines before the specified line and fills the window with the lines following it.

The // and ?? Commands

You can search for a line containing a particular pattern by enclosing the pattern in slashes:

  DB<1> /Find/
18:     # Find all subdirectories of the given directory,

The debugger searches forward from the last displayed line for a line matching the specified pattern. If it finds such a line, the line is displayed.

To search backward for a particular pattern, enclose the pattern in question marks:

  DB<1> ?readsubdirs?
12:                      &readsubdirs($curdir);

This command starts with the last displayed line and searches backward until it finds a line matching the specified pattern.

The S Command

The S command lists all the subroutines in the current file, one subroutine per line:

  DB<> S
main::display
main::followlink
main::menudir
main::readsubdirs

Each subroutine name is preceded by the package name and a single quotation mark.

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Last modified: March, 12, 2019