Set and view environment variables in Linux system

Hello everyone, this is Liang Xu.

Everyone knows that in Linux systems, there are two types of variables: environment variables and shell variables.

Environment variables are globally available in the program and its subprograms, and are often used to store information such as the default text editor or browser, the path of the executable file, and so on. Shell variables are only available in the current Shell and can be used to store information such as the ID of the current user.

So what are environment variables, what are Shell variables, and how to set and view these two variables?

Next, let Liang Xu answer for you:

Environment variable

Environment variables are implemented in the form of key-value pairs. They are variables that are available throughout the system and are inherited by all derived child processes and shells. The names of environment variables are case-sensitive and are usually named in uppercase (MYVAR1, MYVAR2...)

An environment variable with a single value looks like this:

KEY=value1

If you want to assign multiple values to an environment variable, you usually use a colon (:) as the separator. Each key-value pair finally looks like this:

KEY=value1:value2:value3

If the value to be assigned to the environment variable contains spaces, you need to use quotation marks:

KEY="value with spaces"

Shell variables

Shell variables are variables in the Shell that are specifically used to set or define them. Each Shell, such as zsh and bash, has its own set of internal Shell variables. They are usually used to track temporary data, such as the current working directory, and their usage is the same as environment variables.

If you want to use Shell variables as global variables, you can use the exportcommand:

$ export MYVAR=lxlinux.net
$ echo $MYVAR
lxlinux.net
$ env | grep MYVAR
MYVAR=lxlinux.net

Common environment variables and shell variables

Some environment variables and shell variables are very useful and are often quoted. The following are some common environment variables that you may encounter in the future:

variable name	meaning
TERM	This specifies the type of terminal to be simulated when running the Shell. It can simulate different hardware terminals for different operation requirements. However, you usually don't need to worry about this variable.
USER	Currently logged in user
PWD	Current working directory
OLDPWD	On a working directory, this variable is kept by Shell, so that by performing `cd -`back to the previous working directory.
LS_COLORS	This defines the `ls`color output code instructions for the `ls`added color output instruction. This is usually used to distinguish different file types and make the user know the file type and other information at a glance.
MAIL	Path of current user mailbox
PATH	The list of directories that the system will check when looking for instructions. When the user enters an instruction, the system will check the directories in the order of this directory list to find the corresponding executable files.
LANG	Current language and localization settings, including character encoding.
HOME	Home directory of current user
_	The last command executed

In addition to the above environment variables, you may also often encounter the following shell variables:

variable name	meaning
BASHOPTS	The list of options that are enabled when bash is executed, which is helpful for determining whether the shell environment is running as expected.
BASH_VERSION	The running bash version in human readable format
BASH_VERSINFO	The running bash version in machine-readable format
COLUMNS	Used to set the number of wide columns of output information drawn on the screen
DIRSTACK	`pushd`And `popd`command the directory stack available.
HISTFILESIZE	The number of lines of the command history stored in the file. The default is `~/.bash_history`the number of lines in the file.
HISTSIZE	Memory allows the storage of the number of lines of command history, that `histroy`command the number of lines that can be printed out.
HOSTNAME	Hostname of the computer
IFS	Internal field separator, used to separate input on the command line. By default, a space is used as the separator.
PS1	Define the main command prompt. This is used to define the appearance of the command prompt when starting a shell session. PS2 is used to declare a command prompt that spans multiple lines.
SHELLOPTS	You can use `set`the command to set the Shell option.
UID	UID (User ID) of the current user

View shell variables and environment variables

In Linux systems, there are several commands that allow you to view environment variables:

env— This command allows you to run the program in a customized environment without changing the current environment. When used without parameters envcommand, it will print out the current list of environment variables.

printenv — Can print out all or specified environment variables.

set— This command can set or delete Shell variables. When used without parameters setcommand, it will print out a list of all variables, including environment variables and variables, including Shell and the Shell function.

By default, envand the printenvfunction is exactly the same:

$ printenv 
SSH_CONNECTION=10.0.2.2 37182 10.0.2.15 22
LESSCLOSE=/usr/bin/lesspipe %s %s
LANG=C.UTF-8
XDG_SESSION_ID=5
USER=alvin
MYVAR=lxlinux.net
PWD=/home/alvin
HOME=/home/alvin
SSH_CLIENT=10.0.2.2 37182 22
XDG_DATA_DIRS=/usr/local/share:/usr/share:/var/lib/snapd/desktop
SSH_TTY=/dev/pts/0
MAIL=/var/mail/alvin
TERM=xterm-256color
SHELL=/bin/bash
SHLVL=1
LOGNAME=alvin
XDG_RUNTIME_DIR=/run/user/1000
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin
LESSOPEN=| /usr/bin/lesspipe %s
_=/usr/bin/printenv

Only in the more specific features to reflect the envcommand and printenvcommand difference. For example, using the printenvcommand, you can request the value of a single variable:

$ printenv SHELL
/bin/bash
$ printenv HOME
/home/alvin
$ printenv MYVAR
lxlinux.net

env The command can modify the environment in which the program runs by passing a set of variables to the command:

env MYVAR=lxlinux.net command_to_run command_options

printenvAnd envcommand can only print out the environment variable, and if you want to print out a list of all the variables or the Shell function, you can use the setcommand.

$ set
BASH=/bin/bash
BASHOPTS=checkwinsize:cmdhist:complete_fullquote:expand_aliases:extglob:extquote:force_fignore:histappend:interactive_comments:login_shell:progcomp:promptvars:sourcepath
BASH_ALIASES=()
BASH_ARGC=()
BASH_ARGV=()
BASH_CMDS=()
BASH_COMPLETION_VERSINFO=([0]="2" [1]="8")
BASH_LINENO=()
BASH_SOURCE=()
BASH_VERSINFO=([0]="4" [1]="4" [2]="20" [3]="1" [4]="release" [5]="x86_64-pc-linux-gnu")
BASH_VERSION='4.4.20(1)-release'
COLUMNS=140
DIRSTACK=()
EUID=1000
GROUPS=()
HISTCONTROL=ignoreboth
HISTFILE=/home/alvin/.bash_history
HISTFILESIZE=2000
HISTSIZE=1000
HOME=/home/alvin
HOSTNAME=ubuntu-bionic
HOSTTYPE=x86_64
IFS=$' \t\n'
LANG=C.UTF-8
LESSCLOSE='/usr/bin/lesspipe %s %s'
LESSOPEN='| /usr/bin/lesspipe %s'
LINES=35
LOGNAME=alvin
.....

This command will display a large list of all the variables, so you might want to pass to the output of lessthe command.

$ set | less

Set shell variables and environment variables

There are several commands that can set environment variables in Linux system:

set— This command can set or unset Shell variables. When used without parameters setcommand, it will print out a list of all variables, including environment variables and variables, including Shell and the Shell function.

unset — This command can delete shell variables and environment variables.

export -This command can set environment variables.

In order to better understand the difference between Shell variables and environment variables, let's start with setting Shell variables, and then talk about environment variables.

Start by defining a Shell variable in the current session:

$ MYVAR=lxlinux

You can use echo $MYVARto verify that the variable is set:

$ echo $MYVAR
lxlinux

Use printenvcommand to verify that the variable is an environment variable:

$ printenv MYVAR

No output is returned, which means that the MYVAR variable is not an environment variable.

exportCommands can be used to set environment variables. To create an environment variable, just use exportthe Shell command variable exported as environment variables:

$ export MYVAR

You can check with the following statement:

$ printenv MYVAR
lxlinux

Of course, you can also set environment variables with just one line of code:

$ export MYNEWVAR="My New Variable"

But the environment variables created in this way can only be used in the current session. If you open a new Shell session or log out, all the variables will be lost.

We can also restore environment variables to Shell variables, or delete them completely:

The MYVAR variable is defined as an environment variable, and we can restore it to a Shell variable by entering the following code:

$ export -n MYVAR

In this way, the MYVAR variable is no longer an environment variable, but still a Shell variable.

Whether Shell variables or environmental variables, you want to completely remove the variable, you can use the unsetcommand to delete:

$ unset MYVAR

The following statement can be used to verify that the MYVAR variable has been deleted:

$ echo $MYVAR

Since the variable has been deleted, nothing is returned.

Persistence of environment variables

Many programs need to use environment variables to determine the specific execution method, but we do not want to reset important variables every time a new Shell session is started, so we need to write important environment variables into the configuration file.

Shell sessions can be started in different ways, for example, interactive shells connected to the terminal and non-interactive shells not connected to the terminal, as well as login shells and non-login shells. The bash shell varies according to the different startup methods of the session. Read different configuration files.

However, in most Linux distributions, when you start a new Shell session, you generally read environment variables from the following files:

/etc/environment — Use this file to set system-wide environment variables available.

/etc/profile — Whenever bash logs in to Shell, the variables set in this file will be loaded.

~/.bashrc— Shell configuration files specific to each user. For example, if you are using Bash, you can declare variables in it.

If you want to load new environment variables into the current Shell session, you can use the source command:

$ source ~/.bashrc

If you want to set environment variables, you can consider adding it to /etc/profile, /etc/bash.bashrcor /etc/environmentfile.

in conclusion

In this article, we learned about some common environment variables and Shell variables, and learned how to set and view these variables. In fact, these variables have always been in our Shell session and are useful for many programs. There are many common All scenes will reference these variables. Hope this will help your work too. If you still have any questions about these two variables, please leave a message and let me know!

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