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<h4 class="subsection">5.1.7 Breakpoint Command Lists</h4>

<p><a name="index-breakpoint-commands-326"></a>You can give any breakpoint (or watchpoint or catchpoint) a series of
commands to execute when your program stops due to that breakpoint.  For
example, you might want to print the values of certain expressions, or
enable other breakpoints.

     
<a name="index-commands-327"></a>
<a name="index-end_0040r_007b-_0028breakpoint-commands_0029_007d-328"></a>
<dl><dt><code>commands </code><span class="roman">[</span><var>list</var><code>...</code><span class="roman">]</span><dt><code>... </code><var>command-list</var><code> ...</code><dt><code>end</code><dd>Specify a list of commands for the given breakpoints.  The commands
themselves appear on the following lines.  Type a line containing just
<code>end</code> to terminate the commands.

     <p>To remove all commands from a breakpoint, type <code>commands</code> and
follow it immediately with <code>end</code>; that is, give no commands.

     <p>With no argument, <code>commands</code> refers to the last breakpoint,
watchpoint, or catchpoint set (not to the breakpoint most recently
encountered).  If the most recent breakpoints were set with a single
command, then the <code>commands</code> will apply to all the breakpoints
set by that command.  This applies to breakpoints set by
<code>rbreak</code>, and also applies when a single <code>break</code> command
creates multiple breakpoints (see <a href="Ambiguous-Expressions.html#Ambiguous-Expressions">Ambiguous Expressions</a>). 
</dl>

   <p>Pressing &lt;RET&gt; as a means of repeating the last <span class="sc">gdb</span> command is
disabled within a <var>command-list</var>.

   <p>You can use breakpoint commands to start your program up again.  Simply
use the <code>continue</code> command, or <code>step</code>, or any other command
that resumes execution.

   <p>Any other commands in the command list, after a command that resumes
execution, are ignored.  This is because any time you resume execution
(even with a simple <code>next</code> or <code>step</code>), you may encounter
another breakpoint&mdash;which could have its own command list, leading to
ambiguities about which list to execute.

   <p><a name="index-silent-329"></a>If the first command you specify in a command list is <code>silent</code>, the
usual message about stopping at a breakpoint is not printed.  This may
be desirable for breakpoints that are to print a specific message and
then continue.  If none of the remaining commands print anything, you
see no sign that the breakpoint was reached.  <code>silent</code> is
meaningful only at the beginning of a breakpoint command list.

   <p>The commands <code>echo</code>, <code>output</code>, and <code>printf</code> allow you to
print precisely controlled output, and are often useful in silent
breakpoints.  See <a href="Output.html#Output">Commands for Controlled Output</a>.

   <p>For example, here is how you could use breakpoint commands to print the
value of <code>x</code> at entry to <code>foo</code> whenever <code>x</code> is positive.

<pre class="smallexample">     break foo if x&gt;0
     commands
     silent
     printf "x is %d\n",x
     cont
     end
</pre>
   <p>One application for breakpoint commands is to compensate for one bug so
you can test for another.  Put a breakpoint just after the erroneous line
of code, give it a condition to detect the case in which something
erroneous has been done, and give it commands to assign correct values
to any variables that need them.  End with the <code>continue</code> command
so that your program does not stop, and start with the <code>silent</code>
command so that no output is produced.  Here is an example:

<pre class="smallexample">     break 403
     commands
     silent
     set x = y + 4
     cont
     end
</pre>
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