[Linux] When setting environment variables, env, set, export, discard

These are the Bash (or sh) built-in commands that Linux uses the most.

Each command has a slightly different usage.

env : Commands to show, set, or delete environment variables. http://ss64.com/bash/env.html

$ env

Outputs environment variables defined in the current session to the screen.

$ env NAME=VALUE 

Specify a value called VALUE in the environment variable called NAME.

$ env -u NAME

Deletes the NAME environment variable.

set : The command to manage the shell variable in Bash. http://ss64.com/bash/set.html

set NAME=VALUE 
# In Bash, you can omit:
NAME=VALUE

The shell variable is the variable used in the shell script language called Bash, and the environment variable is the variable used by the operating system (for example, PATH).

To turn it off, use the unset command.

export: This command changes the shell variable to the environment variable.

NAME=VALUE
export NAME

The above uses set and export to obtain the same results as the following commands:

env NAME=VALUE

declare: This is a command that specifies the type of shell variable. https://wiki.kldp.org/HOWTO/html/Adv-Bash-Scr-HOWTO/declareref.html

Use to declare the nature of a shell variable as read-only, integer, array, function, etc.

Note