GNU Debugger Commands - dummies

By Emmett Dulaney

The most common way to debug a program in Linux is to run gdb. gdb has a large number of commands, but you need only a few to find the cause of an error quickly. The table lists the commonly used gdb commands.

Common gdb Commands
This Command Does the Following
break NUM Sets a breakpoint at the specified line number, NUM. (The debugger stops at
bt Displays a trace of all stack frames. (This command shows you
the sequence of function calls so far.)
clear FILENAME: NUM Deletes the breakpoint at a specific line number, NUM, in the source file FILENAME. For example, clear
xdraw.c:8 clears
the breakpoint at line 8 of file
continue Continues running the program being debugged. (Use this command
after the program stops due to a signal or breakpoint.)
display EXPR Displays the value of an expression, EXPR (consisting of variables defined in the
program) each time the program stops.
file FILE Loads the specified executable file, FILE, for debugging.
help NAME Displays help on the command named NAME.
info break Displays a list of current breakpoints, including information
on how many times each breakpoint is reached.
info files Displays detailed information about the file being
info func Displays all function names.
info local Displays information about local variables of the current
info prog Displays the execution status of the program being
info var Displays all global and static variable names.
kill Ends the program you’re debugging.
list Lists a section of the source code.
make Runs the make utility to rebuild the executable without leaving
next Advances one line of source code in the current function
without stepping into other functions.
print EXPR Shows the value of the expression EXPR.
quit Quits gdb.
run Starts running the currently loaded executable.
set variable VAR=VALUE Sets the value of the variable VAR to
shell CMD Executes the Unix command CMD,
without leaving gdb.
step Advances one line in the current function, stepping into other
functions, if any.
watch VAR Shows the value of the variable named VAR whenever the value changes.
where Displays the call sequence. Use this command to locate where
your program died.
x/F ADDR Examines the contents of the memory location at address
ADDR in the format specified by the letter F, which
can be o (octal), x (hex), d (decimal), u (unsigned decimal), t
(binary), f (float), a (address), i (instruction), c (char), or s
(string). You can append a letter indicating the size of data type
to the format letter. Size letters are b (byte), h (halfword, 2
bytes), w (word, 4 bytes), and g (giant, 8 bytes). Typically,
ADDR is the name of a variable or pointer.