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Full Discussion: while read loop; scope of variables (shell)
Top Forums Shell Programming and Scripting while read loop; scope of variables (shell) Post 302293625 by mjd_tech on Tuesday 3rd of March 2009 01:32:10 PM
Old 03-03-2009
Ah! (light bulb turns on inside head)
I get it now.
I was thinking the "line" variable would hold the last line read.

so with:
while read line ; do some-command ; done < some-file

"read" tries to read the next line after the end of file, which is null,
and sets "line" to null, and returns "false" so the loop will end.

The "line" variable is not "invisible" outside the loop,
it just looks like that because it ends up null.

Just to make sure,
Code:
while [ "$line" != "Fred" ]; do
    read line
done << EOF
Wilma
Barney
Fred
Betty
EOF

echo "Line = $line"

The result is "Fred" in both bash and dash.

I've used this method before, but never really understood how it worked.

Thanks.

-MD
 

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LEARN ABOUT REDHAT

gets

gets(n) 						       Tcl Built-In Commands							   gets(n)

__________________________________________________________________________________________________________________________________________________

NAME

       gets - Read a line from a channel

SYNOPSIS

       gets channelId ?varName?
_________________________________________________________________

DESCRIPTION

       This command reads the next line from channelId, returns everything in the line up to (but not including) the end-of-line character(s), and
       discards the end-of-line character(s).  If varName is omitted the line is returned as the result of the command.  If varName  is  specified
       then the line is placed in the variable by that name and the return value is a count of the number of characters returned.

       If  end	of file occurs while scanning for an end of line, the command returns whatever input is available up to the end of file.  If chan-
       nelId is in nonblocking mode and there is not a full line of input available, the command returns an empty string and does not consume  any
       input.	If  varName is specified and an empty string is returned in varName because of end-of-file or because of insufficient data in non-
       blocking mode, then the return count is -1.  Note that if varName is not specified then the end-of-file	and  no-full-line-available  cases
       can produce the same results as if there were an input line consisting only of the end-of-line character(s).  The eof and fblocked commands
       can be used to distinguish these three cases.

SEE ALSO

       file(n), eof(n), fblocked(n)

KEYWORDS

       blocking, channel, end of file, end of line, line, nonblocking, read

Tcl									7.5								   gets(n)
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