How to print lines matching pattern or string ?

Consider you have a file called dictionary.txt with a huge collection of words and you only want to view line containing the word "tux". To check each word line by line is a tedious task, instead we use the grep command.

grep command searches line by line for a string or a pattern in a file containing plain-text data and displays the results. This is one of the most powerful and most used Linux commands.The syntax for using grep is

grep [option] pattern filename
grep [option] pattern /path/to/file

Example

grep tux /usr/share/dict/words

will check for lines containing the word "tux" inside the dictionary word file /usr/share/dict/words
and display the result

Patuxent
tux
tuxedo
tuxedoed
tuxedoes
tuxedos
tuxes

Some important options:

-i  : Ignores case sensitivity in both pattern and input file.

-v : Displays lines not containing the pattern.

-c :  Prints the count of matching lines instead of the normal output.

-n :  Prefix each line of output with line number within its input file.

-r :  Read  all files under each directory, recursively.

-x : Performs only exact line matches

--color : Matching patterns displayed colorfully.

egrep and fgrep

egrep and fgrep are two variants of the grep command.  egrep is the same as grep -E, which  Interpret pattern as an extended regular expression. Similarly  fgrep is the same as grep -F and the pattern is interpreted as a list of fixed strings, separated by newlines, any of which is to be matched.