Start process from bash without inheriting file descriptors

Question

I want to start a new process from my bash script which won't inherit parent file descriptors. I can't change the way these file descriptors are created.

Use case: error in application -> error hook -> kill the process and restart it

There is a similar topic for windows start without inheritance of parents file descriptors but it doesn't work for Linux.

Is this even possible in shell?

Thanks

UPDATE

I know that I can close those descriptors myself I just want to make sure that it isn't possible to start child with some magic option to skip copying of file descriptors. (because this option sounds reasonable to me)

score 7 · Answer 1 · edited May 23 '17 at 12:18

7

It is not possible to prevent file descriptor inheritance in general. In C you can use FD_CLOEXEC but even that is only a partial solution as explained here.

Close those file descriptors you do not need:

exec 3>&- #close fd 3.

A longer example:

#! /bin/bash

# Open some file descriptors
for n in $(seq 10 20) ; do
    eval "exec $n>/dev/null"
done

# List them
echo -n "$BASHPID: " ; ls -v /proc/$BASHPID/fd

# Start a sub process without them
(
    # List inherited descriptors
    echo -n "$BASHPID: " ; ls -v /proc/$BASHPID/fd

    # Close all but stdio
    SELF=$BASHPID
    FDS=$(find /proc/$SELF/fd -type l -printf '%f\n')
    # The following will even try to close the fd for the find sub
    # shell although it is already closed. (0: stdin, 1: stdout, 2:
    # stderr, 3: find)
    echo -n 'Closing:'
    for n in $FDS ; do
    if ((n > 2)) ; then
        echo -n " $n"
        eval "exec $n>&-"
    fi
    done
    echo

    # List remaining
    echo -n "$BASHPID: " ; ls -v /proc/$BASHPID/fd

    # Do what you want
)

# Original shell has sill all descriptors
echo -n "$BASHPID: " ; ls -v /proc/$BASHPID/fd

edited May 23 '17 at 12:18

Community

1
1

answered Apr 16 '14 at 09:13

ceving

21,900
13
104
178

But how do you know which descriptors are open? – Roberto Reale Apr 16 '14 at 09:14
1

You have opened them! `ls /proc/$$/fd` might help to remember. – ceving Apr 16 '14 at 09:15
Of course, but what if I want to close *every* file descriptor I opened or inherited? – Roberto Reale Apr 16 '14 at 09:17
@RobertoReale Bash has a `for` loop. – ceving Apr 16 '14 at 09:19
The problem is that `exec 3>&-` does work, but `x=3; exec $x>&-` doesn't. So, how would you write the `for` loop? – Roberto Reale Apr 16 '14 at 09:25
Yes, this will work, I know about it, but do you know whether starting a child without this inheritance is possible? – jakub.petr Apr 16 '14 at 09:26
Note that you should only be working explicitly with file descriptors between 3 and 9 in a `bash` script. Larger numbers may be in use internally by the shell. – chepner Apr 16 '14 at 13:00
@RobertoReale, to close descriptor $x, use `exec {x}>&-`. – Piotr Findeisen Apr 06 '16 at 13:54
1

@ceving, `ls /proc/$$/fd` will list FDs of the top shell process. It won't help if you have subshell that opens something and then forks again. Strange, but this can happen unawarely, e.g. due to pipes. Use `$BASHPID` instead of `$$` in the subshell. – Piotr Findeisen Apr 06 '16 at 13:56
will this also close file descriptors for the parent process? What about sockets? – Arsen Zahray Dec 01 '20 at 10:08

Stuart P. Bentley · Answer 2 · 2014-10-11T23:53:56.540

0

You can write a function to close each of Bash's 10 inheritable file descriptors:

fdcloexec () {
  "$@" 0>&- 1>&- 2>&- 3>&- 4>&- 5>&- 6>&- 7>&- 8>&- 9>&-
}

Then you would call fdcloexec your command here to execute your command without inheriting file descriptors.

edited Oct 11 '14 at 23:53

answered Oct 11 '14 at 23:40

Stuart P. Bentley

10,195
10
55
84

1

Other descriptors (e.g. 13) are also inherited, for example into subshell `... | { ... subshell here ... }`. At least in my bash. – Piotr Findeisen Apr 06 '16 at 13:52

Start process from bash without inheriting file descriptors

UPDATE

2 Answers2