How can I see the exact command line being executed inside some bash instance?

Question

I have a long running bash instance (inside a screen session) that is executing a complex set of commands inside a loop (with each loop doing pipes, redirects, etc).

The long command line was written inside the terminal - it's not inside any script. Now, I know the bash process ID, and I have root access - how can I see the exact command line being executed inside that bash?

Example

bash$ echo $$
1234
bash$ while true ; do \
    someThing | somethingElse 2>/foo/bar | \
    yetAnother ; sleep 600 ; done

And in another shell instance, I want to see the command line executed inside PID 1234:

bash$ echo $$
5678
bash$ su -
sh# cd /proc/1234
sh# # Do something here that will display the string  \
   'while true ; do someThing | somethingElse 2>/foo/bar | \
    yetAnother ; sleep 600 ; done'

Is this possible?

EDIT #1

Adding counter-examples for some answers I've got.

1. About using the cmdline under /proc/PID: that doesn't work, at least not in my scenario. Here's a simple example:

   $ echo $$
   8909
   
   $ while true ; do echo 1 ; echo 2>/dev/null ; sleep 30 ; done
   

   In another shell:

   $ cat /proc/8909/cmdline
   bash
   

2. Using ps -p PID --noheaders -o cmd is just as useless:

   $ ps -p 8909 --no-headers -o cmd
   bash
   

3. ps -eaf is also not helpful:

   $ ps -eaf | grep 8909
   ttsiod    8909  8905  0 10:09 pts/0    00:00:00 bash
   ttsiod   30697  8909  0 10:22 pts/0    00:00:00 sleep 30
   ttsiod   31292 13928  0 10:23 pts/12   00:00:00 grep --color=auto 8909
   

   That is, there's no output of the ORIGINAL command line, which is what I'm looking for - i.e the while true ; do echo 1 ; echo 2>/dev/null ; sleep 30 ; done.

Answer 1

I knew I was grasping at straws, but UNIX never fails!

Here's how I managed it:

bash$ gdb --pid 8909
...
Loaded symbols for /lib/i386-linux-gnu/i686/cmov/libnss_files.so.2
0xb76e7424 in __kernel_vsyscall ()

Then at the (gdb) prompt I ran the command, call write_history("/tmp/foo") which will write this history to the file /tmp/foo.

(gdb) call write_history("/tmp/foo")
$1 = 0

I then detach from the process.

(gdb) detach
Detaching from program: /bin/bash, process 8909

And quit gdb.

(gdb) q

And sure enough...

bash$ tail -1 /tmp/foo
while true ; do echo 1 ; echo 2>/dev/null ; sleep 30 ; done

For easy future re-use, I wrote a bash script, automating the process.

Answer 2

Since the command is still running in screen, its parent bash has not reread any history so:

• reattach to screen
• press ^Z then up arrow
• bonus: wrap the command in single quotes (navigating with ^A^A - because screen(1) - and ^E) and echo + redirect into a file
• fg to pursue command execution

There are caveats, but this is useful enough, most of the time.

Answer 3

I know you found your own answer, but is there a reason you can't do something like this:

(set -x; for f in 1 2 3 4 ; do  echo "$f"; sleep $f; done)

Perhaps you can't co-mingle the output of the actual job and the output from bash showing the currently executing line.

Also, FWIW, if you prefer verbosity, set -o xtrace.