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Finding Patterns in Files

grep (global regular expression print) searches for lines that match a pattern and prints them.

grep "pattern" filename

Example:

grep "Linux" notes.txt

If notes.txt contains:

Learn Linux
Practice daily
Have fun

Output:

Learn Linux

Only lines containing "Linux" are printed.

Flag	Meaning
`-i`	Case-insensitive search
`-v`	Invert match — print lines that do NOT match
`-n`	Show line numbers
`-r`	Recursive — search all files in a directory
`-l`	Show only filenames that contain matches

Search case-insensitively:

grep -i "linux" notes.txt

Print lines that do NOT contain "Linux":

grep -v "Linux" notes.txt

Output:

Practice daily
Have fun

The "pattern" can be a regular expression:

grep "^P" notes.txt    # lines starting with P
grep "y$" notes.txt    # lines ending with y

Search notes.txt for lines containing Linux.

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