invoking bash with the -t option

Question

Can anyone tell me about the -t option when invoking bash?

I found the following handy command to use a lightweight terminal window as an application launcher

urxvt -geometry 80x3 -name 'bashrun' -e sh -c "/bin/bash -i -t"

Through a little investigation of its behavior, I see that the -t essentially makes bash exit after executing one command, but I am curious if there is any more to that specific -t option. I can't actually find any documentation about it, and I've looked through the following docs:

• man bash
• Invoking Bash
• standard bash options

I find it odd that -t is not as clearly laid out as the rest of the options.

Answer 1

You are right that it is amazingly poorly documented. What documentation that there is is quite simple:

-t      Exit after reading and executing one command.

The bash source code is available here. I looked at version 4.2. The short flags handled by bash are listed in flags.c and in there is the relevant line:

{ 't', &just_one_command },

So, -t on the command line sets the variable just_one_command. This variable is used in just one place: it occurs in an if condition at the end of a loop in eval.c:

  if (just_one_command)
    EOF_Reached = EOF;

In other words, if the -t flag is given, then, after the first command is executed, the end-of-file condition is signaled and bash exits.

MORE DETAIL

From eval.c, command line execution in bash appears to be controlled by the function reader_loop:

reader_loop ()
{
  int our_indirection_level;
  COMMAND * volatile current_command;
  USE_VAR(current_command);
  current_command = (COMMAND *)NULL;
  our_indirection_level = ++indirection_level;
  while (EOF_Reached == 0)
    {
      int code;
      code = setjmp_nosigs (top_level);

      [ ... Much code removed ... ]

      if (just_one_command)
        EOF_Reached = EOF;
    }
  indirection_level--;
  return (last_command_exit_value);
}

The loop inside reader_loop continues until it receives the signal EOF_Reached. The sole effect of the -t option is to set this flag at the end of loop which assures that the loop is executed only once.

Answer 2

Using the bash builtin help, i.e. by saying help set, you'd find:

  -t  Exit after reading and executing one command.

Since bash parses a line at a time, you might have multiple commands separated by ; on the same line and those would execute. For example, the following:

set -t; echo Do something; echo do more; echo and more
echo no more

would produce

Do something
do more
and more

as the output. On the other hand, the following:

set -t
echo Do something
echo do more
echo and more
echo no more

would not produce any output.

---

You could look for the documentation in the info page. Say:

info bash "Shell Builtin Commands"

Navigate to * Bourne Shell Builtins:: and search for help.

Answer 3

It is explained in the man page, just difficult to find.

The options not mentioned in the "OPTIONS" block at the beginning are options for set which can be given as options to bash, too:

The options can also be specified as arguments to an invocation of the shell.

-t      Exit after reading and executing one command.