Make a breakpoint when a certain text is written to stdout

stdout_breakpoint.gdb

# Make a breakpoint when a certain text is written to stdout 
# Note: only work with C/C++ binaries
set width 0
set height 0
set verbose off

# Modify this to search for a different string
# Case sensitive
set $STR_SEARCH = "hello world"

# See https://stackoverflow.com/a/25979468
set $exit_syscall = 1
catch syscall write
commands 1
  set $exit_syscall = ! $exit_syscall
  if !$exit_syscall
    continue
  end

  # Disable if you program copies the stdout or stderr fd
  # Check for stdout or stderr fd
  if $rdi != 1 && $rdi != 2
    continue
  end

  echo "Found write at fd:\n"
  p $rdi

  # Calculate string written
  set $written_ptr=(char*)$rsi
  set $written_len=$rdx

  echo Written:\n
  p *$written_ptr@$written_len

  if (char*) strstr(*$written_ptr@$written_len, $STR_SEARCH) != 0
    echo Match found!\n
  else
    continue
    # Comment this to drop to shell on any text written
  end

end

run