If with grep help


 
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# 1  
Old 11-09-2011
If with grep help

Hi guys,

I was wondering if you could help me figure out what's wrong with this script below:
Code:
read username
 
if `grep $username /etc/passwd` > /dev/null
then
   $username >> information_file
else
   echo " "
fi

Apparently command is not found when script is run. Standard Output says the first "if" line is the culprit.

Many thanks!

Last edited by Scott; 11-09-2011 at 01:22 PM.. Reason: Use code tags, please...
# 2  
Old 11-09-2011
Quote:
Originally Posted by jjb1989
Code:
   $username >> information_file

This will attempt to run the value of $username as a command, which is not what you want to do. You probably want to do

Code:
   echo $username >> information_file

You should also validate that $username has a value before you try to use it in a grep.
# 3  
Old 11-09-2011
Code:
 
   echo $username >> information_file

You should also validate that $username has a value before you try to use it in a grep.[/QUOTE]

Thanks for your reply man. Unfortuneatly, Bash still didn't accept the changes. What I want out of this if statement is to check if a username a user typed in can be found in etc/passwd. If yes, then to append that username variable to a text file called information_file.
# 4  
Old 11-09-2011
Quote:
Originally Posted by jjb1989

Thanks for your reply man. Unfortuneatly, Bash still didn't accept the changes. What I want out of this if statement is to check if a username a user typed in can be found in etc/passwd. If yes, then to append that username variable to a text file called information_file.
You need to remove the backticks as well:

Code:
if grep $username /etc/passwd > /dev/null

This User Gave Thanks to mobitron For This Post:
# 5  
Old 11-09-2011
Well, that made all the difference, thanks man! Smilie

Feel stupid now but when should you use back quotes then?
# 6  
Old 11-09-2011
Back quotes turn the output of a program into a string. Use them to do that.
# 7  
Old 11-09-2011
Quote:
Originally Posted by jjb1989
Well, that made all the difference, thanks man! Smilie

Feel stupid now but when should you use back quotes then?
You should use them when you want to pass the output of what is within the quotes to another command, or a variable assignment.

e.g. if you wanted to assign the content of the user's passwd entry to a variable you would do

Code:
passwd_entry=`grep $username /etc/passwd`

The way you were using it, the output of the grep was being passed to the if statement, so the script tried to execute it as a command, which failed.
This User Gave Thanks to mobitron For This Post:
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