Библиотека сайта или "Мой Linux Documentation Project"
written by StИphane Chazelas, and revised by the document author
A command expects the first three file
descriptors to be available. The first, fd
0 (standard input,
is for reading. The other two (fd 1,
stdout and fd 2,
stderr) are for writing.
There is 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
By convention, a command reads its input from fd 0
stdin), prints normal output to fd
stdout), and error ouput to fd 2
stderr). If one of those three fd's is
not open, you may encounter problems:
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
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
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