23.2 env: Run a command in a modified environment

env runs a command with a modified environment. Synopses:

env [option]… [name=value]… [command [args]…]
env

Operands of the form ‘variable=value’ set the environment variable variable to value value. value may be empty (‘variable=’). Setting a variable to an empty value is different from unsetting it. These operands are evaluated left-to-right, so if two operands mention the same variable the earlier is ignored.

Environment variable names can be empty, and can contain any characters other than ‘=’ and ASCII NUL. However, it is wise to limit yourself to names that consist solely of underscores, digits, and ASCII letters, and that begin with a non-digit, as applications like the shell do not work well with other names.

The first operand that does not contain the character ‘=’ specifies the program to invoke; it is searched for according to the PATH environment variable. Any remaining arguments are passed as arguments to that program. The program should not be a special built-in utility (see Special built-in utilities).

Modifications to PATH take effect prior to searching for command. Use caution when reducing PATH; behavior is not portable when PATH is undefined or omits key directories such as /bin.

In the rare case that a utility contains a ‘=’ in the name, the only way to disambiguate it from a variable assignment is to use an intermediate command for command, and pass the problematic program name via args. For example, if ./prog= is an executable in the current PATH:

env prog= true # runs 'true', with prog= in environment
env ./prog= true # runs 'true', with ./prog= in environment
env -- prog= true # runs 'true', with prog= in environment
env sh -c '\prog= true' # runs 'prog=' with argument 'true'
env sh -c 'exec "$@"' sh prog= true # also runs 'prog='

If no command name is specified following the environment specifications, the resulting environment is printed. This is like specifying the printenv program.

For some examples, suppose the environment passed to env contains ‘LOGNAME=rms’, ‘EDITOR=emacs’, and ‘PATH=.:/gnubin:/hacks’:

• Output the current environment.

  $ env | LC_ALL=C sort
  EDITOR=emacs
  LOGNAME=rms
  PATH=.:/gnubin:/hacks
  

• Run foo with a reduced environment, preserving only the original PATH to avoid problems in locating foo.

  env - PATH="$PATH" foo
  

• Run foo with the environment containing ‘LOGNAME=rms’, ‘EDITOR=emacs’, and ‘PATH=.:/gnubin:/hacks’, and guarantees that foo was found in the file system rather than as a shell built-in.

  env foo
  

• Run nemacs with the environment containing ‘LOGNAME=foo’, ‘EDITOR=emacs’, ‘PATH=.:/gnubin:/hacks’, and ‘DISPLAY=gnu:0’.

  env DISPLAY=gnu:0 LOGNAME=foo nemacs
  

• Attempt to run the program /energy/-- (as that is the only possible path search result); if the command exists, the environment will contain ‘LOGNAME=rms’ and ‘PATH=/energy’, and the arguments will be ‘e=mc2’, ‘bar’, and ‘baz’.

  env -u EDITOR PATH=/energy -- e=mc2 bar baz
  

The program accepts the following options. Also see Common options. Options must precede operands.

‘-0’

‘--null’

Output a zero byte (ASCII NUL) at the end of each line, rather than a newline. This option enables other programs to parse the output even when that output would contain data with embedded newlines.

‘-u name’

‘--unset=name’

Remove variable name from the environment, if it was in the environment.

‘-’

‘-i’

‘--ignore-environment’

Start with an empty environment, ignoring the inherited environment.

‘-C dir’

‘--chdir=dir’

Change the working directory to dir before invoking command. This differs from the shell built-in cd in that it starts command as a subprocess rather than altering the shell’s own working directory; this allows it to be chained with other commands that run commands in a different context. For example:

# Run 'true' with /chroot as its root directory and /srv as its working
# directory.
chroot /chroot env --chdir=/srv true
# Run 'true' with /build as its working directory, FOO=bar in its
# environment, and a time limit of five seconds.
env --chdir=/build FOO=bar timeout 5 true

Exit status:

0   if no command is specified and the environment is output
125 if env itself fails
126 if command is found but cannot be invoked
127 if command cannot be found
the exit status of command otherwise