Retired Documents Library Developer
Search
Go to the first, previous, next, last section, table of contents.


Canned Sequences of Commands

Aside from breakpoint commands (see section Breakpoint command lists), GDB provides two ways to store sequences of commands for execution as a unit: user-defined commands and command files.

User-defined commands

A user-defined command is a sequence of GDB commands to which you assign a new name as a command. This is done with the define command. User commands may accept up to 10 arguments separated by whitespace. Arguments are accessed within the user command via $arg0...$arg9. A trivial example:

define adder
  print $arg0 + $arg1 + $arg2

To execute the command use:

adder 1 2 3

This defines the command adder, which prints the sum of its three arguments. Note the arguments are text substitutions, so they may reference variables, use complex expressions, or even perform inferior functions calls.

define commandname
Define a command named commandname. If there is already a command by that name, you are asked to confirm that you want to redefine it. The definition of the command is made up of other GDB command lines, which are given following the define command. The end of these commands is marked by a line containing end.
if
else
Takes a single argument, which is an expression to evaluate. It is followed by a series of commands that are executed only if the expression is true (nonzero). There can then optionally be a line else, followed by a series of commands that are only executed if the expression was false. The end of the list is marked by a line containing end.
while
The syntax is similar to if: the command takes a single argument, which is an expression to evaluate, and must be followed by the commands to execute, one per line, terminated by an end. The commands are executed repeatedly as long as the expression evaluates to true.
document commandname
Document the user-defined command commandname, so that it can be accessed by help. The command commandname must already be defined. This command reads lines of documentation just as define reads the lines of the command definition, ending with end. After the document command is finished, help on command commandname displays the documentation you have written. You may use the document command again to change the documentation of a command. Redefining the command with define does not change the documentation.
dont-repeat
Used inside a user-defined command, this tells GDB that this command should not be repeated when the user hits RET (see section Command syntax).
help user-defined
List all user-defined commands, with the first line of the documentation (if any) for each.
show user
show user commandname
Display the GDB commands used to define commandname (but not its documentation). If no commandname is given, display the definitions for all user-defined commands.
show max-user-call-depth
set max-user-call-depth
The value of max-user-call-depth controls how many recursion levels are allowed in user-defined commands before GDB suspects an infinite recursion and aborts the command.

When user-defined commands are executed, the commands of the definition are not printed. An error in any command stops execution of the user-defined command.

If used interactively, commands that would ask for confirmation proceed without asking when used inside a user-defined command. Many GDB commands that normally print messages to say what they are doing omit the messages when used in a user-defined command.

User-defined command hooks

You may define hooks, which are a special kind of user-defined command. Whenever you run the command `foo', if the user-defined command `hook-foo' exists, it is executed (with no arguments) before that command.

A hook may also be defined which is run after the command you executed. Whenever you run the command `foo', if the user-defined command `hookpost-foo' exists, it is executed (with no arguments) after that command. Post-execution hooks may exist simultaneously with pre-execution hooks, for the same command.

It is valid for a hook to call the command which it hooks. If this occurs, the hook is not re-executed, thereby avoiding infinite recursion.

In addition, a pseudo-command, `stop' exists. Defining (`hook-stop') makes the associated commands execute every time execution stops in your program: before breakpoint commands are run, displays are printed, or the stack frame is printed.

For example, to ignore SIGALRM signals while single-stepping, but treat them normally during normal execution, you could define:

define hook-stop
handle SIGALRM nopass
end

define hook-run
handle SIGALRM pass
end

define hook-continue
handle SIGLARM pass
end

As a further example, to hook at the begining and end of the echo command, and to add extra text to the beginning and end of the message, you could define:

define hook-echo
echo <<<---
end

define hookpost-echo
echo --->>>\n
end

(gdb) echo Hello World
<<<---Hello World--->>>
(gdb)

You can define a hook for any single-word command in GDB, but not for command aliases; you should define a hook for the basic command name, e.g. backtrace rather than bt. If an error occurs during the execution of your hook, execution of GDB commands stops and GDB issues a prompt (before the command that you actually typed had a chance to run).

If you try to define a hook which does not match any known command, you get a warning from the define command.

Command files

A command file for GDB is a text file made of lines that are GDB commands. Comments (lines starting with #) may also be included. An empty line in a command file does nothing; it does not mean to repeat the last command, as it would from the terminal.

You can request the execution of a command file with the source command:

source filename
Execute the command file filename.

The lines in a command file are executed sequentially. They are not printed as they are executed. An error in any command terminates execution of the command file and control is returned to the console.

Commands that would ask for confirmation if used interactively proceed without asking when used in a command file. Many GDB commands that normally print messages to say what they are doing omit the messages when called from command files.

GDB also accepts command input from standard input. In this mode, normal output goes to standard output and error output goes to standard error. Errors in a command file supplied on standard input do not terminate execution of the command file--execution continues with the next command.

gdb < cmds > log 2>&1

(The syntax above will vary depending on the shell used.) This example will execute commands from the file `cmds'. All output and errors would be directed to `log'.

Commands for controlled output

During the execution of a command file or a user-defined command, normal GDB output is suppressed; the only output that appears is what is explicitly printed by the commands in the definition. This section describes three commands useful for generating exactly the output you want.

echo text
Print text. Nonprinting characters can be included in text using C escape sequences, such as `\n' to print a newline. No newline is printed unless you specify one. In addition to the standard C escape sequences, a backslash followed by a space stands for a space. This is useful for displaying a string with spaces at the beginning or the end, since leading and trailing spaces are otherwise trimmed from all arguments. To print ` and foo = ', use the command `echo \ and foo = \ '. A backslash at the end of text can be used, as in C, to continue the command onto subsequent lines. For example,
echo This is some text\n\
which is continued\n\
onto several lines.\n
produces the same output as
echo This is some text\n
echo which is continued\n
echo onto several lines.\n
output expression
Print the value of expression and nothing but that value: no newlines, no `$nn = '. The value is not entered in the value history either. See section Expressions, for more information on expressions.
output/fmt expression
Print the value of expression in format fmt. You can use the same formats as for print. See section Output formats, for more information.
printf string, expressions...
Print the values of the expressions under the control of string. The expressions are separated by commas and may be either numbers or pointers. Their values are printed as specified by string, exactly as if your program were to execute the C subroutine
printf (string, expressions...);
For example, you can print two values in hex like this:
printf "foo, bar-foo = 0x%x, 0x%x\n", foo, bar-foo
The only backslash-escape sequences that you can use in the format string are the simple ones that consist of backslash followed by a letter.


Go to the first, previous, next, last section, table of contents.