Unix Shell Searching Text: grep

1. grep, egrep, fgrep

Three are 2 sets of regular expression format in unix world:
BRE: Basic Regular Expressions
ERE: Extended Regular Expressions

grep is using BRE, egrep is using ERE, fgrep is fixed string match.
Latest POSIX standard merged BRE,ERE,fgrep into grep, with different options, in Ubuntu Linux:

grep -G : basic regular expressions(BRE), this is default
grep -E : extended regular expressions(ERE)
grep -F : fixed string match

2. Use grep:

test:

 #! /bin/bash  
 num_users #output command not found, since   
 echo $PATH   
 #output: /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games  
 PATH=$PATH:$HOME/Desktop/xxdev #Add current path to $PATH  
 echo $PATH #output normal path string + current path  
 #output: /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/home/aubinxia/Desktop/xxdev  
 num_users # ok to run now  
 #output:   
 #+ who  
 #+ wc -l  
 #2  
 #+ set +x  

Use BRE(List all lines containing PATH in file ./test)

 aubinxia@aubinxia-VirtualBox:~/Desktop/xxdev$ grep PATH ./test  
 echo $PATH   
 PATH=$PATH:$HOME/Desktop/xxdev #Add current path to $PATH  
 echo $PATH #output normal path string + current path  

Use ERE, and fixed string:

 aubinxia@aubinxia-VirtualBox:~/Desktop/xxdev$ grep -E PATH ./test  
 echo $PATH   
 PATH=$PATH:$HOME/Desktop/xxdev #Add current path to $PATH  
 echo $PATH #output normal path string + current path  
 aubinxia@aubinxia-VirtualBox:~/Desktop/xxdev$ grep -F PATH ./test  
 echo $PATH   
 PATH=$PATH:$HOME/Desktop/xxdev #Add current path to $PATH  
 echo $PATH #output normal path string + current path  

Use Other options:
-q: quiet, output nothing:

 aubinxia@aubinxia-VirtualBox:~/Desktop/xxdev$ grep -q PATH ./test  
 aubinxia@aubinxia-VirtualBox:~/Desktop/xxdev$  

-l: list all files, instead of outputting the line itself

 aubinxia@aubinxia-VirtualBox:~/Desktop/xxdev$ grep -l PATH ./*  
 ./test  

-v: list all lines which don't match the pattern:

 aubinxia@aubinxia-VirtualBox:~/Desktop/xxdev$ grep -v PATH ./test  
 #! /bin/bash  
 num_users #output command not found, since   
 #output: /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games  
 #output: /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/home/aubinxia/Desktop/xxdev  
 num_users # ok to run now  
 #output:   
 #+ who  
 #+ wc -l  
 #2  
 #+ set +x  

Combine several options:
Use ERE, and only list files themselves:

 aubinxia@aubinxia-VirtualBox:~/Desktop/xxdev$ grep -lE PATH ./test  
 ./test