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Full Discussion: Peculiar behavior due to IFS
Operating Systems Linux Peculiar behavior due to IFS Post 302778321 by alister on Sunday 10th of March 2013 12:53:27 PM
Old 03-10-2013
Quote:
Originally Posted by ravisingh
By this I meant: (for ex.)
Code:
grep us file.txt

How the command line is parsed. Or, in other words, how grep understand what is 1st argument and the 2nd one? Here it isn't considering IFS!
IFS is only used to split the results of unquoted expansions (such as $var, $(cmd)) and by the read command. The shell does not use it during its initial scan of the command line. During that early step, whitespace always delimits words (tokens, to be more precise), regardless of the value of IFS.

With regard to how grep finds its first argument, that's more involved. Once the shell has finished parsing the command line, it forks (or clones) itself. If all goes well, the new subshell calls one of the functions in the exec family with the command to run and a list of arguments to pass to it. This replaces the subshell with the command that was exec'd. The command's arguments are found in the array argv[], with the first argument at argv[1] (argv[0] is the command's name).

The details of creating the process and locating the list of arguments (and the environment) are overseen by the kernel and the c runtime.

Also, should you need to know every last detail of how your shell interprets command lines, you should read your shell's manual page in its entirety. There is also the POSIX standard's documentation, which will give you detailed knowledge of a common UNIX baseline:

POSIX
sh
Shell Command Language
exec family

Those links aren't intended to silence you (your questions are very welcome). I provide them only in case you are not aware of them.

Regards,
Alister

Last edited by alister; 03-10-2013 at 02:00 PM..
 

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URI::URL(3)						User Contributed Perl Documentation					       URI::URL(3)

NAME
URI::URL - Uniform Resource Locators SYNOPSIS
$u1 = URI::URL->new($str, $base); $u2 = $u1->abs; DESCRIPTION
This module is provided for backwards compatibility with modules that depend on the interface provided by the "URI::URL" class that used to be distributed with the libwww-perl library. The following differences exist compared to the "URI" class interface: o The URI::URL module exports the url() function as an alternate constructor interface. o The constructor takes an optional $base argument. The "URI::URL" class is a subclass of "URI::WithBase". o The URI::URL->newlocal class method is the same as URI::file->new_abs. o URI::URL::strict(1) o $url->print_on method o $url->crack method o $url->full_path: same as ($uri->abs_path || "/") o $url->netloc: same as $uri->authority o $url->epath, $url->equery: same as $uri->path, $uri->query o $url->path and $url->query pass unescaped strings. o $url->path_components: same as $uri->path_segments (if you don't consider path segment parameters) o $url->params and $url->eparams methods o $url->base method. See URI::WithBase. o $url->abs and $url->rel have an optional $base argument. See URI::WithBase. o $url->frag: same as $uri->fragment o $url->keywords: same as $uri->query_keywords o $url->localpath and friends map to $uri->file. o $url->address and $url->encoded822addr: same as $uri->to for mailto URI o $url->groupart method for news URI o $url->article: same as $uri->message SEE ALSO
URI, URI::WithBase COPYRIGHT
Copyright 1998-2000 Gisle Aas. perl v5.12.1 2008-04-04 URI::URL(3)
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