env is a
shell command for
Unix and
Unix-like
A Unix-like (sometimes referred to as UN*X, *nix or *NIX) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Uni ...
operating system
An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs.
Time-sharing operating systems scheduler (computing), schedule tasks for ...
s. It is used to either print a list of
environment variable
An environment variable is a user-definable value that can affect the way running processes will behave on a computer. Environment variables are part of the environment in which a process runs. For example, a running process can query the va ...
s 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.
In practice,
env has another common use. It is often used by
shell scripts to launch the correct
interpreter. In this usage, the environment is typically not changed.
History
The version of
env bundled in
GNU coreutils was written by Richard Mlynarik, David MacKenzie, and Assaf Gordon.
It first appeared in
4.4BSD, and is a part of POSIX.1 (with the option only).
GNU's has been extended to handle signals and the current directory.
[ FreeBSD's supports a custom search path. Extensions found in both versions include , for unsetting variables, and , for splitting arguments (mainly in shebang).][
]
Examples
To print out the set of current environment variables:
env
To create a new environment without any existing environment variables for a new shell:
env -i /bin/sh
To execute the X application xcalc and have it appear on a different display (i.e., with a modified environment where the specified environment variable is replaced with the new value):
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:
DISPLAY=foo.bar:1.0 xcalc
env may also be used in the hashbang line of a script to allow the interpreter to be looked up via the PATH. For example, here is the code of a Python script:
#!/usr/bin/env python3
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 (more precisely, env does a system call to execvp, which does the job of locating the interpreter and launching it). 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.
See also
* set
References
External links
*
*
env
��manual page from GNU coreutils.
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{{Core Utilities commands
Standard Unix programs
Unix SUS2008 utilities
Inferno (operating system) commands
IBM i Qshell commands
Environment variables