Which stream does Bash write its prompt to?

Question

I am trying to redirect all output from bash (prompt, user input, results) to a file

Example:

/bin/bash > file.txt 2>&1

I thought that would work, but I'm not getting the prompt. Can anyone tell me what I'm doing wrong?

Answer 1

Bash outputs the prompt only in interactive mode. I.e. it is normally output to the terminal (/dev/tty on linux). That is neither /dev/stdout or /dev/stdin :)

Now, I'm not sure but I can imagine that bash will allow limited interactive mode when there isn't a fully functional tty. In that case I'd expect the prompt to be written to stdout. I haven't tested that.

Nice Proof Of Concept:

(for a in some set of words; do echo $a > /dev/tty; done) 2>&1 > /dev/null

will just output 1..10 as if there wasn't redirection. Like the prompt, output is directly sent to the terminal (which will fail if there isn't one)

HINT: if you wanted everything to be collected look at

• script(1) with scriptreplay ( Capture the execution logs in SSH client? or Scripts shell recording playback - this appears to be somewhat of a FAQ)
• the PROMPT_COMMAND variable which should allow you to print stuff to stdout as a sideeffect of displaying the prompt
• set -o xtrace (a.k.a. set -x, bash -x etc) for general logging of statements

Answer 2

To trick bash into thinking it's in interactive mode (although stdout is not being sent to a terminal) you may use the already mentioned script command.

(
exec 1> >(tee bashlog.txt) 2>&1
script -q /dev/null /bin/bash -l
)

# alternative without script command
(
# bash: no job control in this shell
exec 1> >(tee bashlog.txt) 2>&1
/bin/bash -il
)

Answer 3

The simplest way to do it would be

bash -i >/tmp/logfile 2>&1

Bash will write everything to /tmp/logfile and keep executing commands as you type them, but nothing will be displayed in the terminal. You can make it exit just as you exit your terminal session - by pressing Ctrl+D or typing exit.

Notice that if you run the same thing without stderr redirection, you will only have the greeting message logged to the file, all the rest will work in your current terminal. So the answer to your question about stream to which bash outputs its prompt (and all the following commands) seems to be: stderr.

_{Oh yes, and the -i parameter simply forces bash to run in interactive mode. Don't listen to those people - you don't need any magic tricks to do that ;)}

Answer 4

The prompt is written to stderr as truss (on Solaris here) shows:

$ truss -ft write -p 10501
10501:  write(2, " d", 1)               = 1
10501:  write(2, " a", 1)               = 1
10501:  write(2, " t", 1)               = 1
10501:  write(2, " e", 1)               = 1
10501:  write(2, "\n", 1)               = 1
10521:  write(1, " S a t u r d a y ,   S e".., 46)  = 46
10501:      Received signal #18, SIGCLD [caught]
10501:        siginfo: SIGCLD CLD_EXITED pid=10521 status=0x0000
10501:  write(2, " $  ", 2)             = 2