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Full Discussion: Fetch the value from Here Document
Top Forums Shell Programming and Scripting Fetch the value from Here Document Post 302717839 by Don Cragun on Thursday 18th of October 2012 04:04:47 PM
Old 10-18-2012
Quote:
Originally Posted by adisky123
I have code like
Code:
var="1"
echo << EOF
export `var="2"`
EOF
echo $var

The value of var is printed here is 1 but it should be 2
Any error there?

---------- Post updated at 11:44 AM ---------- Previous update was at 10:33 AM ----------

Also tried
Code:
var="1"
echo var << EOF
echo "2"
EOF
echo $var

That isn't the way echo works and that isn't the way here-documents work.
The echo utility doesn't read standard input, so nothing being done in the here document will be echo'ed by echo.

A here-document operates in the shell execution environment of the command that will be run; not in the current shell execution environment. So, anything you execute in the here-document may affect echo's environment, but it won't be visible in the environment of the shell that ran echo.

The purpose of a here-document is to create text that will be able to be read from the file descriptor specified (fd 0 by default) by the command line that contains the here-document.

If you put the text var=valuein a here-document, the shell doesn't assign value to var; it just makes the stirng "var=value" available to be read from standard input of the command. You can see this if you use cat instead of echo:
Code:
cat <<EOF
var=value
variables are expanded (\$HOME is $HOME)
command substitutions are performed 2+2 is $(echo 2 + 2|bc)
but variables that affect the environment of the shell calling cat cannot be set
EOF

which produces
Code:
var=value
variables are expanded ($HOME is /Users/dwc)
command substitutions are performed 2+2 is 4
but variables that affect the environment of the shell calling cat cannot be set

 

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LEARN ABOUT OSX

ppmtosixel

ppmtosixel(1)						      General Commands Manual						     ppmtosixel(1)

NAME

       ppmtosixel - convert a portable pixmap into DEC sixel format

SYNOPSIS

       ppmtosixel [-raw] [-margin] [ppmfile]

DESCRIPTION

       Reads  a  portable  pixmap  as input.  Produces sixel commands (SIX) as output.	The output is formatted for color printing, e.g. for a DEC
       LJ250 color inkjet printer.

       If RGB values from the PPM file do not have maxval=100, the RGB values are rescaled.  A printer control header and a color assignment table
       begin the SIX file.  Image data is written in a compressed format by default.  A printer control footer ends the image file.

OPTIONS

       -raw   If  specified,  each  pixel  will  be explicitly described in the image file.  If -raw is not specified, output will default to com-
	      pressed format in which identical adjacent pixels are replaced by "repeat pixel" commands.  A raw file is often an order	of  magni-
	      tude larger than a compressed file and prints much slower.

       -margin
	      If  -margin  is not specified, the image will be start at the left margin (of the window, paper, or whatever).  If -margin is speci-
	      fied, a 1.5 inch left margin will offset the image.

PRINTING

       Generally, sixel files must reach the printer unfiltered.  Use the lpr -x option or cat filename > /dev/tty0?.

BUGS

       Upon rescaling, truncation of the least significant bits of RGB values may result in poor color conversion.  If the original PPM maxval was
       greater	than  100,  rescaling  also reduces the image depth.  While the actual RGB values from the ppm file are more or less retained, the
       color palette of the LJ250 may not match the colors on your screen.  This seems to be a printer limitation.

SEE ALSO

       ppm(5)

AUTHOR

       Copyright (C) 1991 by Rick Vinci.

								   26 April 1991						     ppmtosixel(1)
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