written by Stéphane Chazelas, and revised by the document author
A command expects the first three file descriptors to be
available. The first, fd 0 (standard input, stdin), is for
reading. The other two (fd 1, stdout and fd 2, stderr) are for
writing.
There is a stdin, stdout, and a stderr associated with each
command. ls 2>&1 means temporarily connecting the stderr of
the ls command to the same "resource" as the shell's stdout.
By convention, a command reads its input from fd 0 (stdin),
prints normal output to fd 1 (stdout), and error ouput to
fd 2 (stderr). If one of those three fd's is not open, you may
encounter problems:
bash$ cat /etc/passwd >&- cat: standard output: Bad file descriptor
For example, when xterm runs, it first initializes
itself. Before running the user's shell, xterm opens the
terminal device (/dev/pts/<n> or something similar) three
times.
At this point, Bash inherits these three file descriptors, and each
command (child process) run by Bash inherits them in turn, except when
you redirect the command. Redirection means reassigning one of the
file descriptors to another file (or a pipe, or anything
permissible). File descriptors may be reassigned locally (for a
command, a command group, a subshell, a while or if or case or for
loop...), or globally, for the remainder of the shell (using
exec).
ls > /dev/null means running ls with its fd
1 connected to /dev/null.
bash$ lsof -a -p $$ -d0,1,2 COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME bash 363 bozo 0u CHR 136,1 3 /dev/pts/1 bash 363 bozo 1u CHR 136,1 3 /dev/pts/1 bash 363 bozo 2u CHR 136,1 3 /dev/pts/1
bash$ exec 2> /dev/null bash$ lsof -a -p $$ -d0,1,2 COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME bash 371 bozo 0u CHR 136,1 3 /dev/pts/1 bash 371 bozo 1u CHR 136,1 3 /dev/pts/1 bash 371 bozo 2w CHR 1,3 120 /dev/null
bash$ bash -c 'lsof -a -p $$ -d0,1,2' | cat COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME lsof 379 root 0u CHR 136,1 3 /dev/pts/1 lsof 379 root 1w FIFO 0,0 7118 pipe lsof 379 root 2u CHR 136,1 3 /dev/pts/1
bash$ echo "$(bash -c 'lsof -a -p $$ -d0,1,2' 2>&1)" COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME lsof 426 root 0u CHR 136,1 3 /dev/pts/1 lsof 426 root 1w FIFO 0,0 7520 pipe lsof 426 root 2w FIFO 0,0 7520 pipe
This works for different types of redirection.
Exercise: Analyze the following script.
#! /usr/bin/env bash
mkfifo /tmp/fifo1 /tmp/fifo2 while read a; do echo "FIFO1: $a"; done < /tmp/fifo1 & exec 7> /tmp/fifo1 exec 8> >(while read a; do echo "FD8: $a, to fd7"; done >&7)
exec 3>&1 ( ( ( while read a; do echo "FIFO2: $a"; done < /tmp/fifo2 | tee /dev/stderr \ | tee /dev/fd/4 | tee /dev/fd/5 | tee /dev/fd/6 >&7 & exec 3> /tmp/fifo2
echo 1st, to stdout sleep 1 echo 2nd, to stderr >&2 sleep 1 echo 3rd, to fd 3 >&3 sleep 1 echo 4th, to fd 4 >&4 sleep 1 echo 5th, to fd 5 >&5 sleep 1 echo 6th, through a pipe | sed 's/.*/PIPE: &, to fd 5/' >&5 sleep 1 echo 7th, to fd 6 >&6 sleep 1 echo 8th, to fd 7 >&7 sleep 1 echo 9th, to fd 8 >&8
) 4>&1 >&3 3>&- | while read a; do echo "FD4: $a"; done 1>&3 5>&- 6>&- ) 5>&1 >&3 | while read a; do echo "FD5: $a"; done 1>&3 6>&- ) 6>&1 >&3 | while read a; do echo "FD6: $a"; done 3>&-
rm -f /tmp/fifo1 /tmp/fifo2
# For each command and subshell, figure out which fd points to what. # Good luck!
exit 0