14

Is there a way to close all the open file descriptors, without having an explicit list of them beforehand?

4
  • 4
    All of which file descriptors? Every process has some open. Apr 6 '14 at 17:54
  • What are "all the open file descriptors"? 0, 1, and 2? Or do you have many more? If so, where did they come from?
    – U. Windl
    Feb 6 '19 at 15:00
  • Aren't the file descriptors closed automatically when the script finishes?
    – jarno
    Oct 27 '19 at 12:27
  • This is useful to know for the situation when you fork a shell-script, and thus need to close all the parents sockets. Apr 7 at 8:13
18

To answer literally, to close all open file descriptors for bash:

for fd in $(ls /proc/$$/fd); do
  eval "exec $fd>&-"
done

However this really isn't a good idea since it will close the basic file descriptors the shell needs for input and output. If you do this, none of the programs you run will have their output displayed on the terminal (unless they write to the tty device directly). If fact in my tests closing stdin (exec 0>&-) just causes an interactive shell to exit.

What you may actually be looking to do is rather to close all file descriptors that are not part of the shell's basic operation. These are 0 for stdin, 1 for stdout and 2 for stderr. On top of this some shells also seem to have other file descriptors open by default. In bash, for example, you have 255 (also for terminal I/O) and in dash I have 10, which points to /dev/tty rather than the specific tty/pts device the terminal is using. To close everything apart from 0, 1, 2 and 255 in bash:

for fd in $(ls /proc/$$/fd); do
  case "$fd" in
    0|1|2|255)
      ;;
    *)
      eval "exec $fd>&-"
      ;;
  esac
done

Note also that eval is required when redirecting the file descriptor contained in a variable, if not bash will expand the variable but consider it part of the command (in this case it would try to exec the command 0 or 1 or whichever file descriptor you are trying to close).

NOTE: Also using a glob instead of ls (eg /proc/$$/fd/*) seems to open an extra file descriptor for the glob, so ls seems the best solution here.

Update

For further information on the portability of /proc/$$/fd, please see Portability of file descriptor links. If /proc/$$/fd is unavailable, then a drop in replacement for the $(ls /proc/$$/fd), using lsof (if that is available) would be $(lsof -p $$ -Ff | grep f[0-9] | cut -c 2-).

4
  • /proc is only available under Linux.
    – chepner
    Apr 7 '14 at 19:30
  • 4
    @chepner you would be right if you said that /proc/PID/fd is not very portable. But saying that /proc is only available under Linux is not a correct statement.
    – Graeme
    Apr 7 '14 at 19:35
  • Some websites use the <&- form, is it any different/needed? Apr 7 '14 at 21:56
  • @LorenzoPistone, the direction makes no difference when closing a descriptor, so either form can be used.
    – Graeme
    Apr 7 '14 at 22:03
6

In recent versions of Bash (4.1 and onward, year 2009 and later) you can specify the file descriptor to close using a shell variable:

for fd in $(ls /proc/$$/fd/); do
    [ $fd -gt 2 ] && exec {fd}<&-
done

The feature had been in Korn shell already (since 1993?) but apparently took some time to make its way into Bash.

1

Clear all file descriptors except i/o/e of the current shell, but also excludes the ones provided as arguments

clear_fds() {
for fd in $(compgen -G "/proc/$BASHPID/fd/*"); do
    fd=${fd/*\/}
        if [[ ! " $* " =~ " ${fd} " ]]; then
            case "$fd" in
                0|1|2|255)
                ;;
                *)
                    eval "exec $fd>&-"
                    ;;
            esac
        fi
done
}
1

No. The kernel can close only one FD at a time, and bash does not have "group commands" for FDs.

for fd in $(ls -1 /proc/27343/fd); do echo exec $fd">&"-; done

Remove the echo and the " after testing.

If this is not for the shell itself but for a command to be run then you can use nohup.

2
  • 2
    The file descriptor used by >&- cannot be a parameter; the redirection is parsed prior to parameter expansion.
    – chepner
    Apr 7 '14 at 19:35
  • 1
    @chepner nowadays exec {fd}>&- works.
    – jarno
    Oct 27 '19 at 20:42
0

Another way without "eval" at all, is to use the form:

$ exec {var_a}>> file.txt
$ echo $var_a
10
$ ls -l /proc/self/fd/10
l-wx------ 1 0 0 64 Dec 11 18:32 /proc/self/fd/10 -> /run/user/0/tmp/file.txt
$ echo "aaaaa" >&$var_a
$ cat file.txt
aaaaa
$ exec {var_a}>&-
$ ls /proc/self/fd/10
ls: cannot access '/proc/self/fd/10': No such file or directory
2
  • But this doesn't close all file descriptors. Dec 11 '18 at 18:04
  • Indeed. You'll have to exec in a loop like explained in the previous replies... It was just to point the fact that "eval" is not mandatory here and that we can keep the same logic to close it than to open it... Man pages are really not clear about this...
    – David DG
    Dec 11 '18 at 18:11

This site is temporarily in read only mode and not accepting new answers.

Not the answer you're looking for? Browse other questions tagged .