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Full Discussion: Help with retaining variable scope
Top Forums Shell Programming and Scripting Help with retaining variable scope Post 302555938 by radoulov on Friday 16th of September 2011 06:58:25 AM
Old 09-16-2011
Quote:
Originally Posted by bakunin
I respectfully disagree with radoulov. The reason, why your value is not retained is because it is incremented in a subshell.
Hi bakunin,
if I'm not missing something, in the OP's example the value of the variable is not incremented in a subshell
(unless executed by the Bourne shell, because it seems that sh uses a subshell for redirected loops):

Code:
while read ...
Value=`expr $Value + $Count`
...
echo "Value=$Value"

Something like:

Code:
$ unset v
$ while read ; do v=$REPLY; done<<<1
$ echo $v
1

The OP is verifying the value of the variable inside the script, not outside of it.

Your cat ... | while read ... example demonstrates another KornShell feature: the last builtin command in a pipeline being executed in the parent shell.

Consider the following examples:

Code:
$ echo ok > infile
$ sh -c 'cat infile | while read v; do myvar=$v;done; echo $myvar'

$ bash -c 'cat infile | while read v; do myvar=$v;done; echo $myvar'

$ ksh -c 'cat infile | while read v; do myvar=$v;done; echo $myvar'
ok

This is because of the previously mentioned KornShell feature.

And this is the OP's case:

Code:
$ echo ok > infile
$ sh -c 'while read v; do myvar=$v;done<infile; echo $myvar'

$ ksh -c 'while read v; do myvar=$v;done<infile; echo $myvar'
ok
$ bash -c 'while read v; do myvar=$v;done<infile; echo $myvar'
ok

In this case the old Bourne shell behaves differently.

Regards
radoulov

P.S. Actually this aspect of some implementations of the Bourne shell is covered in the FAQ.
Last edited by radoulov; 09-16-2011 at 09:33 AM.. Reason: Clarification.
 

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LEARN ABOUT REDHAT

script

SCRIPT(1)						    BSD General Commands Manual 						 SCRIPT(1)

NAME

     script -- make typescript of terminal session

SYNOPSIS

     script [-a] [-f] [-q] [-t] [file]

DESCRIPTION

     Script makes a typescript of everything printed on your terminal.	It is useful for students who need a hardcopy record of an interactive
     session as proof of an assignment, as the typescript file can be printed out later with lpr(1).

     If the argument file is given, script saves all dialogue in file.	If no file name is given, the typescript is saved in the file typescript.

     Options:

     -a      Append the output to file or typescript, retaining the prior contents.

     -f      Flush output after each write. This is nice for telecooperation: One person does `mkfifo foo; script -f foo' and another can super-
	     vise real-time what is being done using `cat foo'.

     -q      Be quiet.

     -t      Output timeing data to standard error. This data contains two fields, separated by a space. The first field indicates how much time
	     elapsed since the previous output. The second field indicates how many characters were output this time. This information can be used
	     to replay typescripts with realistic typing and output delays.

     The script ends when the forked shell exits (a control-D to exit the Bourne shell (sh(1)), and exit, logout or control-d (if ignoreeof is not
     set) for the C-shell, csh(1)).

     Certain interactive commands, such as vi(1), create garbage in the typescript file.  Script works best with commands that do not manipulate
     the screen, the results are meant to emulate a hardcopy terminal.

ENVIRONMENT

     The following environment variable is utilized by script:

     SHELL  If the variable SHELL exists, the shell forked by script will be that shell. If SHELL is not set, the Bourne shell is assumed. (Most
	    shells set this variable automatically).

SEE ALSO

     csh(1) (for the history mechanism), replay(1).

HISTORY

     The script command appeared in 3.0BSD.

BUGS

     Script places everything in the log file, including linefeeds and backspaces.  This is not what the naive user expects.

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