Bash-Beginners-Guide

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Bash Guide for Beginners michel ~> ls -l /dev/std* lrwxrwxrwx 1 root root 17 Oct 2 07:46 /dev/stderr -> ../proc/self/fd/2 lrwxrwxrwx 1 root root 17 Oct 2 07:46 /dev/stdin -> ../proc/self/fd/0 lrwxrwxrwx 1 root root 17 Oct 2 07:46 /dev/stdout -> ../proc/self/fd/1 michel ~> ls -l /proc/self/fd/[0-2] lrwx------ 1 michel michel 64 Jan 23 12:11 /proc/self/fd/0 -> /dev/pts/6 lrwx------ 1 michel michel 64 Jan 23 12:11 /proc/self/fd/1 -> /dev/pts/6 lrwx------ 1 michel michel 64 Jan 23 12:11 /proc/self/fd/2 -> /dev/pts/6 Note that each process has its own view of the files under /proc/self, as it is actually a symbolic link to /proc/. You might want to check info MAKEDEV and info proc for more information about /proc subdirectories and the way your system handles standard file descriptors for each running process. When excuting a given command, the following steps are excuted, in order: • If the standard output of a previous command is being piped to the standard input of the current command, then /proc//fd/0 is updated to target the same anonymous pipe as /proc/ date Fri Jan 24 11:05:50 CET 2003 michel ~> at 1107 warning: commands will be executed using (in order) a) $SHELL b) login shell c)/bin/sh at> ls -l /proc/self/fd/ > /var/tmp/fdtest.at at> job 10 at 2003-01-24 11:07 michel ~> cat /var/tmp/fdtest.at total 0 lr-x------ 1 michel michel 64 Jan 24 11:07 0 -> /var/spool/at/!0000c010959eb (deleted) l-wx------ 1 michel michel 64 Jan 24 11:07 1 -> /var/tmp/fdtest.at l-wx------ 1 michel michel 64 Jan 24 11:07 2 -> /var/spool/at/spool/a0000c010959eb lr-x------ 1 michel michel 64 Jan 24 11:07 3 -> /proc/21949/fd And one with cron: michel ~> crontab -l Chapter 8. Writing interactive scripts 100
Bash Guide for Beginners # DO NOT EDIT THIS FILE - edit the master and reinstall. # (/tmp/crontab.21968 installed on Fri Jan 24 11:30:41 2003) # (Cron version -- $Id: chap8.xml,v 1.9 2006/09/28 09:42:45 tille Exp $) 32 11 * * * ls -l /proc/self/fd/ > /var/tmp/fdtest.cron michel ~> cat /var/tmp/fdtest.cron total 0 lr-x------ 1 michel michel 64 Jan 24 11:32 0 -> pipe:[124440] l-wx------ 1 michel michel 64 Jan 24 11:32 1 -> /var/tmp/fdtest.cron l-wx------ 1 michel michel 64 Jan 24 11:32 2 -> pipe:[124441] lr-x------ 1 michel michel 64 Jan 24 11:32 3 -> /proc/21974/fd 8.2.3.2. Redirection of errors From the previous examples, it is clear that you can provide input and output files for a script (see Section 8.2.4 for more), but some tend to forget about redirecting errors - output which might be depended upon later on. Also, if you are lucky, errors will be mailed to you and eventual causes of failure might get revealed. If you are not as lucky, errors will cause your script to fail and won't be caught or sent anywhere, so that you can't start to do any worthwhile debugging. When redirecting errors, note that the order of precedence is significant. For example, this command, issued in /var/spool ls -l * 2> /var/tmp/unaccessible-in-spool will redirect standard output of the ls command to the file unaccessible-in-spool in /var/tmp. The command ls -l * > /var/tmp/spoollist 2>&1 will direct both standard input and standard error to the file spoollist. The command ls -l * 2 >& 1 > /var/tmp/spoollist directs only the standard output to the destination file, because the standard error is copied to standard output before the standard output is redirected. For convenience, errors are often redirected to /dev/null, if it is sure they will not be needed. Hundreds of examples can be found in the startup scripts for your system. Bash allows for both standard output and standard error to be redirected to the file whose name is the result of the expansion of FILE with this construct: &> FILE This is the equivalent of > FILE 2>&1, the construct used in the previous set of examples. It is also often combined with redirection to /dev/null, for instance when you just want a command to execute, no matter what output or errors it gives. 8.2.4. File input and output 8.2.4.1. Using /dev/fd The /dev/fd directory contains entries named 0, 1, 2, and so on. Opening the file /dev/fd/N is equivalent to duplicating file descriptor N. If your system provides /dev/stdin, /dev/stdout and Chapter 8. Writing interactive scripts 101
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Bash Guide for Beginners Machtelt G
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Bash Guide for Beginners Table of C
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Table of Contents Glossary C.......
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Introduction 1. Why this guide? The
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Revision 1.6 2005-03-01 Revised by:
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Bash Guide for Beginners See Chapte
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Bash Guide for Beginners mia:~> cat
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Bash Guide for Beginners When invok
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More information about the file des
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Built-in commands are necessary to
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• Brace expansion • Tilde expan
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A number of methods can be used; on
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Bash Guide for Beginners stop in ea
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Chapter 2. Writing and debugging sc
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the documentation of your favorite
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Bash Guide for Beginners Comments a
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Bash Guide for Beginners + set +x I
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Chapter 3. The Bash environment In
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Bash Guide for Beginners local pid
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MOZILLA_FIVE_HOME=/usr/lib/mozilla
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Bash Guide for Beginners franky ~>
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the GNU Awk programming language is
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Bash Guide for Beginners 3.2.4. Res
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LINES MACHTYPE MAILCHECK OLDPWD OPT
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two is the second positional parame
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Bash Guide for Beginners 3.3.1. Why
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A correctly-formed brace expansion
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Page 73 and 74: sandy ~> sed -n '/a text/,/This/p'
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Page 123 and 124: ;; 2) $FORTUNE startrek ;; 3) $FORT
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Send and receive mail. man Read man
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Bash Guide for Beginners rsh RPM Pa
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W Bash Guide for Beginners w Show w
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Index A aliases Section 3.5.1 ANSI-
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Bash Guide for Beginners Section 3.
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J Bash Guide for Beginners K kill k
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eadonly Section 10.1.3 regular expr
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Bash Guide for Beginners vi(m) Sect
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