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
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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.
1 #! /usr/bin/env bash 2 3 mkfifo /tmp/fifo1 /tmp/fifo2 4 while read a; do echo "FIFO1: $a"; done < /tmp/fifo1 & exec 7> /tmp/fifo1 5 exec 8> >(while read a; do echo "FD8: $a, to fd7"; done >&7) 6 7 exec 3>&1 8 ( 9 ( 10 ( 11 while read a; do echo "FIFO2: $a"; done < /tmp/fifo2 | tee /dev/stderr \ 12 | tee /dev/fd/4 | tee /dev/fd/5 | tee /dev/fd/6 >&7 & exec 3> /tmp/fifo2 13 14 echo 1st, to stdout 15 sleep 1 16 echo 2nd, to stderr >&2 17 sleep 1 18 echo 3rd, to fd 3 >&3 19 sleep 1 20 echo 4th, to fd 4 >&4 21 sleep 1 22 echo 5th, to fd 5 >&5 23 sleep 1 24 echo 6th, through a pipe | sed 's/.*/PIPE: &, to fd 5/' >&5 25 sleep 1 26 echo 7th, to fd 6 >&6 27 sleep 1 28 echo 8th, to fd 7 >&7 29 sleep 1 30 echo 9th, to fd 8 >&8 31 32 ) 4>&1 >&3 3>&- | while read a; do echo "FD4: $a"; done 1>&3 5>&- 6>&- 33 ) 5>&1 >&3 | while read a; do echo "FD5: $a"; done 1>&3 6>&- 34 ) 6>&1 >&3 | while read a; do echo "FD6: $a"; done 3>&- 35 36 rm -f /tmp/fifo1 /tmp/fifo2 37 38 39 # For each command and subshell, figure out which fd points to what. 40 # Good luck! 41 42 exit 0 |