Concept

Origins#

The environment variables in Linux originated from the design requirements of early UNIX systems. Through environment variables, users and processes can dynamically influence system behavior without modifying the system’s core code. This flexibility and configurability make environment variables a key part of Linux system and application configurations.

In UNIX, each process has its own environment (i.e., a set of environment variables), which are passed from the parent process to the child process at startup. The most typical example is when a user logs into the system, the Shell process started by the system inherits a set of default environment variables, such as the user’s home directory, Shell type, etc. Users can modify or add environment variables based on this, affecting the behavior of programs or processes they start.

Concept Corrections#

Some books and articles may divide environment variables into global environment variables and local environment variables, but this classification is inaccurate, because there is no definition of these two categories in official documentation, not even in English names. Therefore, to avoid logical problems in semantics, this article will not classify environment variables as above.

《Linux Command Line and Shell Scripting Bible》

The section on environment variables in this book is problematic, not a translation issue, but the original version has problems.

Printing Env Vars#

Command: printenv#

Without arguments, it prints all environment variables.

printenv | head -n 3

SHELL=/bin/bash
PWD=/home/kuga
LOGNAME=kuga

Prints a specific environment variable.

printenv PATH

/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

Command: env#

Without arguments, it prints all environment variables.

env | head -n 3

SHELL=/bin/bash
PWD=/home/kuga
LOGNAME=kuga

Command: echo#

Prints a specified variable, which can be an environment variable or a Shell variable.

echo $HOME

/home/kuga

Customizing Shell Vars#

Defines a Shell variable named soda with the value green.

soda=green

echo $soda

green

printenv#

printenv soda

Outputs nothing, because soda is not an environment variable, it’s just a Shell variable.

Command Group#

(echo $soda)

green

Modify in Command Group#

(soda=yellow; echo $soda); echo $soda

yellow
green

Subshell modifications do not affect external data.

Creating a Bash Instance#

bash

echo $soda

Outputs nothing, which is the basic scope of a regular Shell variable.

Customizing Env Vars#

Environment variables can be created by exporting Shell variables using the export command, as defined in the official manual.

Mark each name to be passed to child processes in the environment.

Translation: Mark the variable name to be passed to child processes in the environment.

Variables can be defined and exported simultaneously, or separately.

export soda=green

soda=green; export soda;

printenv#

After being converted to an environment variable, printenv can normally print it.

printenv soda

green

Command Group#

(printenv soda)

green

Modify in Command Group#

(soda=yellow; echo $soda); echo $soda

yellow
green

Similarly, modifying environment variables within a subshell does not affect external data.

Creating a Bash Instance#

bash -c "printenv soda"

green

Environment variables are accessible in a new Bash instance.

Get All Export Variables#

Without arguments, or using -p, all export variables can be printed.

export

export | grep soda

declare -x soda="green"

Cancel Export Env Vars#

Using the -n option can cancel export, making it a regular Shell variable again.

export -n soda

printenv soda; echo $soda

printenv has no output, echo outputs normally, because soda is no longer an environment variable.

Deleting Variables#

Below will delete the entire variable, whether it’s an environment variable or a Shell variable.

unset soda

echo $soda

Command: declare#

Used to declare variables and attributes. If no names are given, then display the values of variables instead.

Declare variables and give them attributes. If no names are given, then display the values of variables instead.

Common parameters:

• -i：Declares the variable as an integer.
• -r：Declares the variable as read-only.
• -x：Exports the variable as an environment variable.
• -p：Displays the declaration and current value of the variable.

Without Arguments#

The effect is the same as a regular Shell variable.

declare soda="green"

echo $soda

Display Var Declaration#

declare -p soda

declare -- soda="green"

Display All Vars Declaration#

declare -p

Declare Env Vars#

declare -x soda="green"

printenv soda

green

The definition of soda can also be seen in export.

export | grep soda

Diff Between -x and –#

It’s not hard to see that the declaration symbols can be used to distinguish different types of variables.

• -x：Declaration of environment variables.
• --：Declaration of regular Shell variables.

declare -p soda

declare -x soda="green"

After canceling export.

export -n soda

declare -p soda

declare -- soda="green"

Built-in Cmd Manual#

Some built-in commands cannot be viewed using man, but can be viewed using the help command or the --help option.

export#

type -a export

export is a shell builtin

help export

export --help

declare#

Same as above

unset#

Same as above