env - Linux

Overview

The env command in Linux is used to display, modify, or run commands in a modified environment without affecting the existing environment. It’s commonly used to manipulate environment variables for specific script executions or command-line operations. This tool can craft different environments for different requirements, making it highly effective in testing and development scenarios.

Syntax

The basic syntax of the env command is:

env [OPTIONS]... [-] [NAME=VALUE]... [COMMAND [ARG]...]
  • [OPTIONS]: One or more options altering the behavior of the command.
  • [-]: An optional dash that indicates the end of options.
  • [NAME=VALUE]: Zero or more environment variable settings which are passed to the command.
  • [COMMAND [ARG]…]: An optional command followed by its arguments to run in the modified environment.

Options/Flags

  • -i, --ignore-environment: Start with an empty environment, ignoring the existing environment variables.
  • -0, --null: Output line endings with null characters instead of newlines (useful with xargs -0).
  • -u, --unset=NAME: Remove variable NAME from the environment, if it exists.
  • --help: Display a help message and exit.
  • --version: Output version information and exit.

Examples

  1. Displaying Current Environment Variables

    env

    This command lists all the environment variables for the current session.

  2. Running a Command With a Modified Environment

    env PATH=$HOME/bin:$PATH MY_VAR=production some-command

    Run some-command in an environment where PATH includes $HOME/bin and MY_VAR is set to “production”.

  3. Running a Command With a Clean Environment

    env -i some-command

    Executes some-command with an entirely empty environment.

Common Issues

  • Variables Not Set: Users often forget to format environment variable assignments correctly (NAME=VALUE). Ensure there are no spaces around the = character.
  • Transient Changes: Remember that env changes are temporary and only apply to the command run with env, not affecting the parent shell environment.

Integration

env can be effectively combined with shell scripts or other Linux commands to manage environment-specific tasks. For example, setting up a cron job that needs a particular PATH:

* * * * * env PATH=/usr/local/bin:$PATH /path/to/script.sh

This cron job runs script.sh with an adjusted PATH, ensuring it has access to the necessary executables.

  • printenv: Prints part or all of the current environment.
  • set: Used to set the value of shell options and positional parameters.
  • export: Used to pass environment variables to all child processes.

For further reading on environment variables and command usage, check the Linux man pages or the GNU coreutils online documentation. Additionally, exploring scripting tutorials will give practical insights and extended applications of the env command.