Purpose of <> Post: 302607470

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Top Forums UNIX for Dummies Questions & Answers Purpose of <> Post 302607470 by drl on Wednesday 14th of March 2012 01:48:05 PM

03-14-2012

Registered User

Hi.

The <> re-direction operator is also available in ksh and zsh.

The serial port example makes it useful ... cheers, drl

---------- Post updated at 12:48 ---------- Previous update was at 12:27 ----------

Hi.

Associating a file descriptor with an existing file does not destroy it:

Code:

#!/usr/bin/env bash

# @(#) s3	Demonstrate <> re-direction operator.

pe() { for _i;do printf "%s" "$_i";done; printf "\n"; }
pl() { pe;pe "-----" ;pe "$*"; }
db() { ( printf " db, ";for _i;do printf "%s" "$_i";done;printf "\n" ) >&2 ; }
db() { : ; }
C=$HOME/bin/context && [ -f $C ] && . $C

echo first time > f
pl " Initial content of file f:"
cat f

pl " Associate descriptor 4 with file f, show file:"
exec 4<>f
cat f
ls -lgG f
read -u 4 line
pe " variable line read from FD 4 as \"$line\""

pl " Write to f:"
echo hi >f
ls -lgG f

pl " Read from f:"
cat <f

exit 0

producing:

Code:

% ./s3

Environment: LC_ALL = C, LANG = C
(Versions displayed with local utility "version")
OS, ker|rel, machine: Linux, 2.6.26-2-amd64, x86_64
Distribution        : Debian GNU/Linux 5.0.8 (lenny) 
bash GNU bash 3.2.39

-----
 Initial content of file f:
first time

-----
 Associate descriptor 4 with file f, show file:
first time
-rw-r--r-- 1 11 Mar 14 12:46 f
 variable line read from FD 4 as "first time"

-----
 Write to f:
-rw-r--r-- 1 3 Mar 14 12:46 f

-----
 Read from f:

hi

( I corrected my earlier post. )

Best wishes ... cheers, drl

( edit 1: addition )

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LEARN ABOUT OPENSOLARIS

endusershell

getusershell(3C)					   Standard C Library Functions 					  getusershell(3C)

NAME

       getusershell, setusershell, endusershell - get legal user shells

SYNOPSIS

       #include <unistd.h>

       char *getusershell(void);

       void setusershell(void);

       void endusershell(void);

DESCRIPTION

       The  getusershell()  function  returns  a  pointer  to  a  legal  user  shell  as defined by the system manager in the file /etc/shells. If
       /etc/shells does not exist, the following locations of the standard system shells are used in its place:

	 /bin/bash		    /bin/csh
	 /bin/jsh		    /bin/ksh
	 /bin/ksh93		    /bin/pfcsh
	 /bin/pfksh		    /bin/pfsh
	 /bin/sh		    /bin/tcsh
	 /bin/zsh		    /sbin/jsh
	 /sbin/pfsh		    /sbin/sh
	 /usr/bin/bash		    /usr/bin/csh
	 /usr/bin/jsh		    /usr/bin/ksh
	 /usr/bin/ksh93 	    /usr/bin/pfcsh
	 /usr/bin/pfksh 	    /usr/bin/pfsh
	 /usr/bin/sh		    /usr/bin/tcsh
	 /usr/bin/zsh		    /usr/sfw/bin/zsh
	 /usr/xpg4/bin/sh

       The getusershell() function opens the file /etc/shells, if it exists, and returns the next entry in the list of shells.

       The setusershell() function rewinds the file or the list.

       The endusershell() function closes the file, frees any memory used by getusershell() and setusershell(), and rewinds the file /etc/shells.

RETURN VALUES

       The getusershell() function returns a null pointer on EOF.

BUGS

       All information is contained in memory that may be freed with a call to endusershell(), so it must be copied if it is to be saved.

NOTES

       Restricted shells should not be listed in /etc/shells.

SunOS 5.11							    1 Nov 2007							  getusershell(3C)

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