Unix description

stdin (0)

Default input source, usually the keyboard

stdout (1)

Default output destination, usually the screen

stderr (2)

Standard error output channel, used for error messages

>

redirects stdout to a file (overwrites file)

>>

Appends the stdout to a file

<

Redirects a file into stdin

2>

Redirects stderr to a file (overwrites file)

2>>

Appends stderr to a file

> file 2> &1

Redirects both stdout and stderr to the same file

/dev/null

A special file that discards all data written to it (“bit bucket”).

| (pipe operator)

Sends stdout of one command into stdin of another

head/tail

Display the first or last lines of a file

cut

Extrafcts specific columns or charcters from each line

tr

Translates or deletes characters (e.g., lowercase to uppercase).

sort

Sorts lines of text (with options like - r for reverse)

sed

Steam editor for filtering and transforming text

grep

Searches text using patterns or regular expressions

test command

Evaluates conditions and returns 0 (true) or 1 (false)

-f

Tests if a regular file exists

-d

Tests if a directory exists

-z

Checks if a string is empty

= / !=

Tests for string equality or inequality

-eq / -It / -gt

Tests for a string equality or inequality

if-then-else

Conditional statement that executes commands based on a test

case

Pattern-matching control structure for multiple options

while loop

Repeats commands while a condition is true

for loop

Iterates over a list of values

&&

Logical AND: RUNS SECOND COMMAND IF FIRST SUCCEEDS

||

Logical OR: runs second command if first fails

function_name() {}

Defines a resuable set of commands

return

Exits a function and returns a value

find [path] [criteria] [action]

Searches files and performs actions

-name “pattern”

Search by filename.

-type f / -type d

Search for files or directories

-mtime +30

Finds files modified more than 30 days ago.

-size +1M

Finds files larger than 1MB

-exec command {} ;

Executes a command on each file found

-delete

Deletes matching files

read

Reads input from the user or a file

$#

Number of positional parameters (arguments)

$0

Name of the script

$1, $2, …

Positional parameters passed to the script