GDB lets you attach command list to breakpoints and watchpoints, the syntax of a command list:

    (gdb) commands breakpoints-number
    ...list-of-commands
    ...list-of-commands
    end

For example, consider the following code:

    1 int main(void)
    2 {
    3     int i;
    4 
    5     for (i = 1; i < 100; )
    6         i += i;
    7     printf("i: %d\n", i);
    8     return 0;
    9 }

we'd like to check the values i whenever we hit the breakpoint, so we add the following patch:

    (gdb) break 6
    Breakpoint 1 at 0x55555555465b: file main.c, line 6.

    (gdb) commands 1
    Type commands for breakpoint(s) 1, one per line.
    End with a line saying just "end".
    >echo value of i: \n
    >print i
    >continue
    >end

    (gdb)

We use the echo command to print some message, use print command to print value of i, and use continue command to let gdb continue executing when hits the breakpoint.

When we run the program, the output is equal to add the debug statement printf("value of i:\n %d\n", i); after line 6, but even better because you needn't to delete it or comment it after debuging, you just do things in GDB!