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Perl Debugger Stepping Commands

Classic feature of any debugger worth its name is the ability to execute a program one statement at a time. This is called stepping

The s Command

Command s executes a single statement of your program and stops before the next statement:

  DB<2> s
main::(debugtest:4):        $curdir = "";

This command executes one statement of your program and then displays the next statement to be executed. If the statement executed needs to read from the standard input file, the debugger waits until the input is provided before displaying the next line to execute.

TIP If you have forgotten which line is the next line to execute (because, for example, you have displayed lines using the l command), you can list the next line to execute using the L command:
DB<2> L 
3: $dircount = 0;
The L command lists the last lines executed by the program. It also lists any breakpoints and line actions that have been defined for particular lines. Breakpoints and line actions are discussed later today.

If the statement executed by the s command calls a subroutine, the Perl debugger enters the subroutine but does not execute any statements in it. Instead, it stops at the first executable statement in the subroutine and displays it. For example, if the following is the current line:

main::(debugtest:12):                      &readsubdirs($curdir);

specifying the s command tells the Perl debugger to enter readsubdirs and display the following, which is the first executable line of readsubdirs:

main::readsubdirs(debugtest:27):      local ($dirname) = @_;

The s command assumes that you want to debug the subroutine you have entered. If you know that a particular subroutine works properly and you don't want to step through it one statement at a time, use the n command, described in the following section.

The n Command

The n command, like the s command, executes one line of your program and displays the next line to be executed:

  DB<2> n
main::(debugtest:5):        while (1) {

The n statement, however, does not enter any subroutines. If the statement executed by n contains a subroutine call, the subroutine is executed in its entirety. After the subroutine is executed, the debugger displays the line immediately following the call.

For example, if the current line is

main::(debugtest:12):                      &readsubdirs($curdir);

the n command tells the debugger to execute readsubdirs and then display the next line in the program, which is

main::(debugtest:13:):             }

Combining the use of s and n ensures that the debugger examines only the subroutines you want to see.

The Perl debugger does not enable you to enter any library functions. You can enter only subroutines that you have created yourself or that have been created previously and added to a subroutine library

The f command

The f command tells the Perl debugger to execute the remainder of the statements in the current subroutine and then display the line immediately after the subroutine call. This is useful when you are looking for a bug and have determined that the current subroutine does not contain the problem.

The Carriage-Return Command

If you are stepping through a program using s or n, you can save yourself some typing by just pressing Enter when you want to execute another statement. When you press Enter, the debugger repeats the last s or n command executed.

For example, to step from line 5 to line 7, you can use the s command as usual:

  DB<3> s
main::(debugtest:7):              if ($curdir eq "") {

(Line 6 is skipped because it contains no executable statements.) To execute line 7, you can now just press Enter:

main::(debugtest:8):              print ("Enter directory to list:\n");
Pressing Enter has no effect if you have not specified any s or n commands.

The r Command

If you are inside a subroutine and decide that you no longer need to step through it, you can tell the Perl debugger to finish executing the subroutine and return to the statement after the subroutine call. To do this, use the r command:

  DB<4> r
main::(debugtest:13:):             }

The statement displayed by the debugger is the first statement following the call to the subroutine.



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