To set an environment variable once, use the export command in the prompt, not in a shell script:

$ export THEVAR=/example

The variable will be set for the rest of the shell session or until unset.

To set an environment variable everytime, use the export command in the .bashrc file (or the appropriate initialization file for your shell).

To set an environment variable from a script, use the export command in the script, and then source the script. If you execute the script it will not work.

For an explanation of the difference between sourcing and executing see this answer:

• What is the difference between executing a Bash script vs sourcing it?

I suspect you want to have shell variables to have a limited scope, rather than environment variables. Environment variables are a list of strings passed to commands when they are executed.

In

var=value echo whatever

You're passing the var=value string to the environment that echo receives. However, echo doesn't do anything with its environment list¹ and anyway in most shells, echo is built in and therefore not executed.

If you had written

var=value sh -c 'echo "$var"'

That would have been another matter. Here, we're passing var=value to the sh command, and sh does happen to use its environment. Shells convert each² of the variables they receive from their environment to a shell variable, so the var environment variable sh receives will be converted to a $var variable, and when it expands it in that echo command line, that will become echo value. Because the environment is by default inherited, echo will also receive var=value in its environment (or would if it were executed), but again, echo doesn't care about the environment.

Now, if as I suspect, what you want is to limit the scope of shell variables, there are several possible approaches.

Portably (Bourne and POSIX):

(var=value; echo "1: $var"); echo "2: $var"

The (...) above starts a sub-shell (a new shell process in most shells), so any variable declared there will only affect that sub-shell, so I'd expect the code above to output "1: value" and "2: " or "2: whatever-var-was-set-to-before".

With most Bourne-like shells (see List of shells that support `local` keyword for defining local variables), you can use functions and the "local" builtin:

f() {
  local var
  var=value
  echo "1: $var"
}
f
echo "2: $var"

With zsh, you can use anonymous functions which like normal functions can have a local scope:

(){ local var=value; echo "1: $var"; }; echo "2: $var"

or:

function { local var=value; echo "1: $var"; }; echo "2: $var"

With bash and zsh (but not ash, pdksh or AT&T ksh), this trick also works:

var=value eval 'echo "1: $var"'; echo "2: $var"

A variant that works in a few more shells (dash, mksh, yash) but not zsh (unless in sh/ksh emulation):

var=value command eval 'echo "1: $var"'; echo "2: $var"

(using command in front of a special builtin (here eval) in POSIX shells removes their specialness (here that variables assignments in front of them remain in effect after they have returned))

With the fish shell, you can make variables local to a begin..end block:

begin
  set -l var value
  echo 1: $var
end
echo 2: $var

With mksh (and soon zsh), you can abuse the ${|cmd} construct which can also have a local scope making sure it expands to nothing by making sure you don't set $REPLY within:

${|local var=value; echo "$var"}; echo "$var"

^{¹ Stricktly speaking, that's not completely true. Several implementations will care about the localisation environment variables (LANG, LOCPATH, LC_*...), the GNU implementation cares about the POSIXLY_CORRECT environment variable (compare env echo --version with env POSIXLY_CORRECT=1 echo --version on a GNU system).}

^{² well only (some of) the ones that are valid variable names in the syntax of the particular shell. For instance env ++=foo 1x=bar 3=qwe '#=rty' IFS=asd é=x sh -c 'shell code' will not create ++, 1x, 3, # shell variables as those are not valid variable names (though # and 3 are non-variable shell parameters), and don't import IFS from the environment as that would be source of security vulnerabilities; for é, YMMV.}

Solution: In order to export and keep environment variables persistent on linux you should export variables decelration into one of the following files: ~/.bash_profile / ~/. bash_login / ~/.profile.

When bash is invoked as an interactive/non-interactive login shell, it first reads and executes commands from the file /etc/profile (if exists) and after looks for these following files ( in that order and do the same) ~/.bash_profile, ~/. bash_login, ~/.profile.

Example: adding secret token into my user profile.

cat << End >> ~/.profile
export SECRET_TOKEN=abc123!@#
End

output:

echo $SECRET_TOKEN
abc123!@#

To set permanent environment variables in latest Ubuntu versions (from 14.04 and above) add the variables to /etc/environment. For that follow the below instructions,

Open the terminal and run

sudo -H gedit /etc/environment

the provide your password, then in the prompted text file

then add the variables like

ANT_HOME="/opt/ANT/"

Sample of the /etc/environment is given below

PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games"
JAVA_HOME="/usr/lib/jvm/java-8-oracle/"
AXIS2_HOME="/opt/axis2-1.7.4/"
ANT_HOME="/opt/apache-ant-1.9.7/"

don't forget to logout and login again to enable the environment variables.

You say that

I am always using export command to set environment variable

By the way you worded that, it sounds like you are really trying to ask how do you make an environmental variable persists. To do that would require you to place your export VAR="foo" statement in your $HOME/.bash_profile file (if you are using bash). If you want that environmental variable to persist for all users but root, then add it to /etc/profile. If you want it added for the root user too, then set it in /root/.bash_profile .

This will work for all login shells where bash is the shell of choice. For non login shells, you need to use .bashrc. I have no insights to offer for other shells :D

How about exporting the variable, but only inside the subshell?:

(export FOO=bar && somecommand someargs | somecommand2)

Keith has a point, to unconditionally execute the commands, do this:

(export FOO=bar; somecommand someargs | somecommand2)