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Full Discussion: Effect of using eval to execute a command as opposed to writing it on the commandline
Top Forums Shell Programming and Scripting Effect of using eval to execute a command as opposed to writing it on the commandline Post 302431030 by cfajohnson on Sunday 20th of June 2010 03:07:42 PM
Old 06-20-2010
Quote:
Originally Posted by daudiam
Code:
cmd='date | wc' or cmd="date | wc"
$cmd

If this script is executed, an error is generated. The reason written was that "The execution fails because the pipe is not expanded and is passed to date as an argument". What is meant by expansion of pipe. When we execute date | wc on the command line, it goes fine. then | is not treated as an argument. Why?

If I write
Code:
cmd='date | wc' or cmd="date | wc"
eval $cmd

It works. How?

The shell has already tokenized the command line and performed any redirection before the variable is expanded.

When you use eval, the expansion is done first, then the shell parses the result.
This User Gave Thanks to cfajohnson For This Post:
daudiam
 

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LEARN ABOUT DEBIAN

el

el(1)							      General Commands Manual							     el(1)

NAME

       el -- program to make a tuned shell-command for Oneliner

SYNOPSIS

       el  [-acdfhioxV]   [--command cmd]  [--args n]  [--format fmt]  [--execute]  [--inpipe buf]  [--outpipe buf]  [--serial]  [--help]  [--ver-
       sion]  [--debug]

DESCRIPTION

       This manual page documents briefly the el command.

       This manual page was written for the Debian distribution because the original program does not have a manual page.  Instead, it	has  docu-
       mentation in the GNU	  Info format; see below.

       el  is  a program that You can make a tuned shell-command for Oneliner with.  This command can send S-exp to Emacs, and this can make Emacs
       evaluate it, too.  In other words, you can execute Emacs's function from shell-commands.  And you can make Emacs work in closer cooperation
       with your favorite shell-commands.

OPTIONS

       These  programs	follow	the  usual  GNU  command  line	syntax, with long options starting with two dashes (`-').  A summary of options is
       included below.	For a complete description, see the Info files.

       -c cmd		--command cmd
		 cmd means a function of Elisp.  Command line arguments and data from stdin means arguments of cmd.  If you specify  the  only	-c
		 switch, el output one S-exp by one line of stdin.

       --a n	       --args n
		 Specifies the maximum number of arguments of a Elisp function with -c switch.

       -f fmt		--format fmt
		 Allows you to use format string of printf for making a S-exp.

       -x	    --execute
		 Requests to Oneliner to evaluate the S-exp to use *Oneliner auto-eval* buffer.

       -i buf		--inpipe buf
		 Gets input from pipe-buffer. You can specify a number of pipe-buffer, too.

       -o buf		--outpipe buf
		 Puts output to pipe-buffer. You can specify a number of pipe-buffer, too.

       -s	    --serial
		 Makes el to serialize multiple lines to one line.

       -h	    --help
		 Display help message.

       -V	    --version
		 Display version identifiers.

       -d	    --debug
		 Enable debugging state.

SEE ALSO

       The programs are documented fully by Oneliner(Shell-mode hooks for Oneliners)	   available via the Info system.

AUTHOR

       This  manual  page was written by OHURA Makoto <ohura@debian.org> for the Debian system (but may be used by others).  Permission is granted
       to copy, distribute and/or modify this document under the terms of the GNU General Public License, Version 2 any later version published by
       the Free Software Foundation.

       On Debian systems, the complete text of the GNU General Public License can be found in /usr/share/common-licenses/GPL.

																	     el(1)
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