env is a shell command for Unix and Unix-like operating systems. It is used to either print a list of environment variables or run another utility in an altered environment without having to modify the currently existing environment. Using env, variables may be added or removed, and existing variables may be changed by assigning new values to them.
To print out a list of all environment variables, simply run env without any arguments:
To clear the environment (creating a new environment without any existing environment variables) for a new shell:
env -i /bin/sh
env DISPLAY=foo.bar:1.0 xcalc
Note that this use of env is often unnecessary since most shells support setting environment variables in front of a command:
#!/usr/bin/env python2 print "Hello World."
In this example, /usr/bin/env is the full path of the env command. The environment is not altered.
Note that it is possible to specify the interpreter without using env, by giving the full path of the python interpreter. A problem with that approach is that on different computer systems, the exact path may be different. By instead using env as in the example, the interpreter is searched for and located at the time the script is run. This makes the script more portable, but also increases the risk that the wrong interpreter is selected because it searches for a match in every directory on the executable search path. It also suffers from the same problem in that the path to the env binary may also be different on a per-machine basis.