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Top Forums Shell Programming and Scripting Effect of using eval to execute a command as opposed to writing it on the commandline Post 302430951 by daudiam on Sunday 20th of June 2010 02:43:14 AM
Old 06-20-2010
Effect of using eval to execute a command as opposed to writing it on the commandline

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?

I understand (hopefully) how eval works in 2 passes i.e. the command first gets evaluated and then executed.
 

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libapache2-mod-perl2-2.0.7::docs::api::Apache2::Command(User Contributed Perl Documentlibapache2-mod-perl2-2.0.7::docs::api::Apache2::Command(3pm)

NAME
Apache2::Command - Perl API for accessing Apache module command information Synopsis use Apache2::Module (); use Apache2::Command (); my $module = Apache2::Module::find_linked_module('mod_perl.c'); for (my $cmd = $module->cmds; $cmd; $cmd = $cmd->next) { $cmd->args_how(); $cmd->errmsg(); $cmd->name(); $cmd->req_override(); } Description "Apache2::Command" provides the Perl API for accessing Apache module command information API
"Apache2::Command" provides the following functions and/or methods: "args_how" What the command expects as arguments: $how = $cmd->args_how(); obj: $cmd ( "Apache2::Command object" ) ret: $how ( "Apache2::Const :cmd_how constant" ) The flag value representing the type of this command (i.e. "Apache2::Const::ITERATE", "Apache2::Const::TAKE2"). since: 2.0.00 "errmsg" Get usage message for that command, in case of syntax errors: $error = $cmd->errmsg(); obj: $cmd ( "Apache2::Command object" ) ret: $error ( string ) The error message since: 2.0.00 "name" Get the name of this command: $name = $cmd->name(); obj: $cmd ( "Apache2::Command object" ) ret: $name ( string ) The command name since: 2.0.00 "next" Get the next command in the chain of commands for this module: $next = $cmd->next(); obj: $cmd ( "Apache2::Command object" ) ret: $next ( "Apache2::Command object" ) Returns the next command in the chain for this module, "undef" for the last command. since: 2.0.00 "req_override" What overrides need to be allowed to enable this command: $override = $cmd->req_override obj: $cmd ( "Apache2::Command object" ) ret: $override ( "Apache2::Const :override constant" ) The bit mask representing the overrides this command is allowed in (i.e "Apache2::Const::OR_ALL"/"Apache2::Const::ACCESS_CONF"). since: 2.0.00 For example: use Apache2::Const -compile => qw(:override); $cmd->req_override() & Apache2::Const::OR_AUTHCFG; $cmd->req_override() & Apache2::Const::OR_LIMIT; See Also mod_perl 2.0 documentation. Copyright mod_perl 2.0 and its core modules are copyrighted under The Apache Software License, Version 2.0. Authors The mod_perl development team and numerous contributors. perl v5.14.2 2011-02-08 libapache2-mod-perl2-2.0.7::docs::api::Apache2::Command(3pm)
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