chapter: I/O Redirection Detailed

Author: pmarinov

index.ptree

@@ -120,6 +120,7 @@ pages/bibliography.poly.pm
   pages/awk_micro_primer.poly.pm
   pages/parsing_pathnames.poly.pm
   pages/exit_codes.poly.pm
+  pages/io_redirection_de.poly.pm
 }
 
 }

pages/io_redirection_de.poly.pm

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+#lang pollen
+
+◊page-init{}
+◊define-meta[page-title]{I/O Redirection Detailed}
+◊define-meta[page-description]{A Detailed Introduction to I/O and I/O Redirection}
+
+written by Stéphane Chazelas, and revised by the document author
+
+A command expects the first three file descriptors to be
+available. The first, ◊code{fd 0} (standard input, stdin), is for
+reading. The other two (◊code{fd 1}, stdout and ◊code{fd 2}, stderr) are for
+writing.
+
+There is a stdin, stdout, and a stderr associated with each
+command. ◊command{ls 2>&1} means temporarily connecting the stderr of
+the ◊command{ls} command to the same "resource" as the shell's stdout.
+
+By convention, a command reads its input from ◊code{fd 0} (stdin),
+prints normal output to ◊code{fd 1} (stdout), and error ouput to
+◊code{fd 2} (stderr). If one of those three fd's is not open, you may
+encounter problems:
+
+◊example{
+bash$ cat /etc/passwd >&-
+cat: standard output: Bad file descriptor
+}
+
+For example, when ◊command{xterm} runs, it first initializes
+itself. Before running the user's shell, ◊command{xterm} opens the
+terminal device (◊fname{/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
+◊command{exec}).
+
+◊command{ls > /dev/null} means running ◊command{ls} with its ◊code{fd
+1} connected to ◊fname{/dev/null}.
+
+◊example{
+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.
+
+◊example{
+#! /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
+}