Pipe characters, redirects and environment variables

Input and Output Redirection

Input redirection is the import of a file into a command, while output redirection is the writing of data information that would otherwise be output to the screen to a specified file.

• Standard input redirection (STDIN, file descriptor 0): input from the keyboard by default, but also from other files or commands.

• Standard output redirection (STDOUT, file descriptor 1): output to the screen by default.

• Error output redirection (STDERR, file descriptor 2): default output to screen.

File descriptor 1 can be omitted for standard output mode in redirection, while file descriptor 2 is mandatory for error output mode.

Input redirection

Symbols and their effects

Symbols	Role
Command < file	Use file as standard input for commands
Command << delimiter	Reads from standard input and does not stop until it meets a delimiter
command < file 1 > file 2	use file 1 as standard input to the command and standard output to file 2

Output redirection

Symbols	Effects
Command > File	Redirects the standard output to a file (clears the data of the original file)
Command 2> File	Redirects the error output to a file (clears the data of the original file)
Command >> File	Redirects standard output to a file (appends to the back of the original content)
Command >> File	Redirects error output to a file (appends to the back of the original)
Command >> File 2>&1 or Command &>> File	Writes standard output to a file with error output (appends to the back of the original)

Pipeline command character

The pipe character can be entered by pressing Shift + backslash () on the keyboard and is executed as "Command A | Command B". The Pipeline command takes the information that would have been output to the screen by the previous command and uses it as standard input for the next command.

# Output the number of lines for the user who is banned from logging in
grep /sbin/nologin /etc/passwd | wc -l

# Search for processes related to bash
ps aux | grep bash

Command characters can be infinite combinations, for example: command A | command B | command C | ......

A particularly apt analogy was once made by a student from the north-east: treating pipe characters like an assembly line operation is the same as having a barbecue, i.e. the first person is responsible for cutting the meat, the second for skewering it, the third for grilling it, and the final result is delivered to the user.

Wildcards on the command line

Wildcards and their meanings

Wildcards	Meaning
*	Any character
?	single arbitrary character
[a-z]	a single lowercase letter
[A-Z]	single uppercase letter
[a-Z]	single letter
[0-9]	single digit
[[:alpha:]]	arbitrary letters
[[:upper:]]	any upper case letter
[[:lower:]]	any lowercase letter
[[:digit:]]	all digits
[[:alnum:]]	any letter plus a number
[[:punct:]]	punctuation

Example.

# Match files in the /dev directory that start with sda
ls -l /dev/sda*

Commonly used escape characters

• Backslash (): makes a variable after a backslash a mere character.

• single quotes (' '): escapes all variables in it to mere strings.

• Double quotes (" "): retains the attributes of the variables in it without escaping them.

• backquote ( \ ): returns the result of executing the command in it.

Important environment variables

In Linux, it is a convention that variable names are generally in upper case and commands are in lower case.

The 10 most important environment variables in Linux

Variable names	Role
HOME	The user's home directory (i.e. home directory)
SHELL	The name of the shell interpreter the user is using
HISTSIZE	The number of command history records output
HISTFILESIZE	Number of command history records saved
MAIL	path to save mail
LANG	System language, language name
RANDOM	Generate a random number
PS1	The prompt for the Bash interpreter
PATH	Defines the path where the interpreter searches for commands to be executed by the user
EDITOR	The user's default text editor