####Search for a string in a file
####This is the basic usage of grep command. It searches for the given string in the specified file.
####This searches for the string "Error" in the log file and prints all the lines that has the word "Error".
grep "Error" logfile.txt

####Searching for a string in multiple files.
####This is also the basic usage of the grep command. You can manually specify the list of files you want to search or you can specify a file pattern (use regular expressions) to search for.
grep "string" file1 file2
grep "string" file_pattern

####Case insensitive search
####The -i option enables to search for a string case insensitively in the give file. It matches the words like "UNIX", "Unix", "unix".
grep -i "UNix" file.txt

####Specifying the search string as a regular expression pattern.
####This will search for the lines which starts with a number.
grep "^[0-9].*" file.txt

####Checking for the whole words in a file.
####By default, grep matches the given string/pattern even if it found as a substring in a file. The -w option to grep makes it match only the whole words.
grep -w "world" file.txt

####Displaying the lines before the match.
####Some times, if you are searching for an error in a log file; it is always good to know the lines around the error lines to know the cause of the error.
####This will prints the matched lines along with the two lines before the matched lines.
grep -B 2 "Error" file.txt

####Displaying the lines after the match.
####This will display the matched lines along with the three lines after the matched lines.
grep -A 3 "Error" file.txt

####Displaying the lines around the match
####This will display the matched lines and also five lines before and after the matched lines.
grep -C 5 "Error" file.txt

####Searching for a string in all files recursively
####You can search for a string in all the files under the current directory and sub-directories with the help -r option.
grep -r "string" *

####Inverting the pattern match
####You can display the lines that are not matched with the specified search string pattern using the -v option.
grep -v "string" file.txt

####Displaying the non-empty lines
####You can remove the blank lines using the grep command.
grep -v "^$" file.txt

####Displaying the count of number of matches.
####We can find the number of lines that matches the given string/pattern
grep -c "string" file.txt

####Display the file names that matches the pattern.
####We can just display the files that contains the given string/pattern.
grep -l "string" *

####Display the file names that do not contain the pattern.
####We can display the files which do not contain the matched string/pattern.
grep -L "string" *

####Displaying only the matched pattern.
####By default, grep displays the entire line which has the matched string. We can make the grep to display only the matched string by using the -o option.
grep -o "string" file.txt

####Displaying the line numbers.
####We can make the grep command to display the position of the line which contains the matched string in a file using the -n option
grep -n "string" file.txt

####Displaying the position of the matched string in the line
####The -b option allows the grep command to display the character position of the matched string in a file.
grep -o -b "string" file.txt

####Matching the lines that start with a string
####The ^ regular expression pattern specifies the start of a line. This can be used in grep to match the lines which start with the given string or pattern.
grep "^start" file.txt

####Matching the lines that end with a string
####The $ regular expression pattern specifies the end of a line. This can be used in grep to match the lines which end with the given string or pattern.
grep "end$" file.txt