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Full Discussion: if statement problem
Top Forums Shell Programming and Scripting if statement problem Post 302578507 by Chubler_XL on Thursday 1st of December 2011 06:20:01 PM
Old 12-01-2011
Corona688's grep -q solution is the best, but so you understand how things work I'll try and explain how one gets to it.

1. call grep, and throw away it's output and then examine return status:
Code:
grep "^$username:" /etc/passwd > /dev/null
if [ $? -eq 0 ]
then
     # Found string
    ...
fi

Grep (and most unix commands) set exit status ($?) to zero on sucess (ie expresion matches), and non-zero for failure (expression not found or other errors, eg could not open file)


2. Grep supports a -q option (be quiet and just set exit status), so we can simplify to:
Code:
grep -q "^$username:" /etc/passwd
if [ $? -eq 0 ]
then
     # Found string
    ...
fi


3. if runs command and executes block when command result is zero ( [ is just a shortcut for the test command). So we can use if to directly test the grep result:
Code:
if grep -q "^$username:" /etc/passwd
then
    # Found string
    ...
fi

This User Gave Thanks to Chubler_XL For This Post:
 

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Mail::Mbox::MessageParser::Grep(3)			User Contributed Perl Documentation			Mail::Mbox::MessageParser::Grep(3)

NAME
Mail::Mbox::MessageParser::Grep - A GNU grep-based mbox folder reader SYNOPSIS
#!/usr/bin/perl use Mail::Mbox::MessageParser; my $filename = 'mail/saved-mail'; my $filehandle = new FileHandle($filename); my $folder_reader = new Mail::Mbox::MessageParser( { 'file_name' => $filename, 'file_handle' => $filehandle, 'enable_grep' => 1, } ); die $folder_reader unless ref $folder_reader; # Any newlines or such before the start of the first email my $prologue = $folder_reader->prologue; print $prologue; # This is the main loop. It's executed once for each email while(!$folder_reader->end_of_file()); { my $email = $folder_reader->read_next_email(); print $email; } DESCRIPTION
This module implements a GNU grep-based mbox folder reader. It can only be used when GNU grep is installed on the system. Users must not instantiate this class directly--use Mail::Mbox::MessageParser instead. The base MessageParser module will automatically manage the use of grep and non-grep implementations. METHODS AND FUNCTIONS The following methods and functions are specific to the Mail::Mbox::MessageParser::Grep package. For additional inherited ones, see the Mail::Mbox::MessageParser documentation. $ref = new( { 'file_name' => <mailbox file name>, 'file_handle' => <mailbox file handle> }); <file_name> - The full filename of the mailbox <file_handle> - An opened file handle for the mailbox The constructor for the class takes two parameters. The file_name parameter is the filename of the mailbox. The file_handle argument is the opened file handle to the mailbox. Returns a reference to a Mail::Mbox::MessageParser object, or a string describing the error. BUGS
No known bugs. Contact david@coppit.org for bug reports and suggestions. AUTHOR
David Coppit <david@coppit.org>. LICENSE
This software is distributed under the terms of the GPL. See the file "LICENSE" for more information. HISTORY
This code was originally part of the grepmail distribution. See http://grepmail.sf.net/ for previous versions of grepmail which included early versions of this code. SEE ALSO
Mail::Mbox::MessageParser perl v5.12.1 2009-08-09 Mail::Mbox::MessageParser::Grep(3)
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