When working with your server through a shell session, there are many pieces of information that your shell compiles to determine its behavior and access to resources. Some of these settings are contained within configuration settings and others are determined by user input. One way that the shell keeps track of many of of these settings is through an area it maintains called the environment. The environment is an area that the shell builds every time that it starts a session that contains variables that define system properties.
Environmental variables are variables that are defined for the current shell and are inherited by any child shells or processes. Environmental variables are used to pass information into processes that are spawned from the shell. An environment variable are represent as key-value pairs.
# printenv HOSTNAME=myhostname SELINUX_ROLE_REQUESTED= TERM=xterm SHELL=/bin/bash HISTSIZE=1000 SSH_CLIENT=172.16.51.15 61242 22 SELINUX_USE_CURRENT_RANGE= QTDIR=/usr/lib64/qt-3.3 QTINC=/usr/lib64/qt-3.3/include SSH_TTY=/dev/pts/0 USER=root ...
To set an existing variable only for current shell
VARNAME="my value"
Make sure not to add extra spaces before or after Equals Sign(=)
export VARNAME="my value"
export MYVAR3=value3 export MYVAR4=1004
The /etc/environment file is only used for setting environment variables.
Alternately, add to directory
/etc/profile initializes variables for login shells only. It is also used to run scripts and can be used by all Bourne shell compatible shells.
Instead of etc/profile, consider creating a new shell script under directory /etc/profile.d/. These files will not be modified on an upgrade.
| System Variable | Meaning | To View Variable Value Type |
|---|---|---|
| BASH_VERSION | Holds the version of this instance of bash. | echo $BASH_VERSION |
| HOSTNAME | The name of the your computer. | echo $HOSTNAME |
| CDPATH | The search path for the cd command. | echo $CDPATH |
| HISTFILE | The name of the file in which command history is saved. | echo $HISTFILE |
| HISTFILESIZE | The maximum number of lines contained in the history file. | echo $HISTFILESIZE |
| HISTSIZE | The number of commands to remember in the command history. The default value is 500. | echo $HISTSIZE |
| HOME | The home directory of the current user. | echo $HOME |
| IFS | The Internal Field Separator that is used for word splitting after expansion and to split lines into words with the read builtin command. The default value is <space><tab><newline>. |
echo $IFS |
| LANG | Used to determine the locale category for any category not specifically selected with a variable starting with LC_. | echo $LANG |
| PATH | The search path for commands. It is a colon-separated list of directories in which the shell looks for commands. | echo $PATH |
| PS1 | Your prompt settings. | echo $PS1 |
| TMOUT | The default timeout for the read builtin command. Also in an interactive shell, the value is interpreted as the number of seconds to wait for input after issuing the command. If not input provided it will logout user. |
echo $TMOUT |
| TERM | Your login terminal type. | echo $TERM export TERM=vt100 |
| SHELL | Set path to login shell. | echo $SHELL |
| DISPLAY | Set X display name | echo $DISPLAY export DISPLAY=:0.1 |
| EDITOR | Set name of default text editor. | export EDITOR=/usr/bin/vim |
The Linux shell prompt is set up with environmental variable PS1. By default this variable displays hostname and current working directory. You can easily customize your prompt to display information important to you. The default value is \s-\v\$ .
To display current prompt
$ echo $PS1 \u@\h \W]\$
Some prompt switches
\d : the date in “Weekday Month Date” format (e.g., “Tue May 26”)
\H : the hostname
\t : the current time in 24-hour HH:MM:SS format
\u : the username of the current user
\w : the current working directory, with $HOME abbreviated with a tilde
Example:
PS1="\d \H \t \u \w"
One environment variable is called http_proxy. It allows you to connect text based session and/or applications via the proxy server. In this variable we indicate the proxy server IP address (URL) and port values. This variable is almost used by utilities such as elinks, lynx, wget, curl and others commands. You may create a file called /etc/profile.d/proxy.sh to hold these environmental variables.
Examples:
export http_proxy=http://server-ip:port/ export http_proxy=http://127.0.0.1:3128/ export http_proxy=http://proxy-server.mycorp.com:3128/
If the proxy server requires a username and password then add these to the URL. For example, to include the username foo and the password bar:
export http_proxy=http://foo:bar@server-ip:port/ export http_proxy=http://foo:bar@127.0.0.1:3128/ export http_proxy=http://USERNAME:PASSWORD@proxy-server.mycorp.com:3128/
The PATH is an environmental variable that tells the shell which directories to search for executable files in response to commands issued by a user. A user's PATH consists of a series of colon-separated absolute paths that are stored in plain text files.
adding to the PATH variable:
Add /home/mypath to beginning of PATH
PATH=/home/mypath:$PATH
Add /home/mypath to end of PATH
PATH=$PATH:/home/mypath
Using export to make changes to PATH for current shell and all processes started from current shell
export PATH=$PATH:/home/mypath
To set variables permanently for all your future bash sessions, add export statement to your .bashrc file in your $HOME directory.
vi /home/myuser/.bashrc