env Overview
Environment Variables are dynamic 'name=value' pairs that contain information needed for the operating system to function. Programs refer to these variables when they run to determine their behavior, paths, and more. The `env` command is used to manage these environment variables.
The Importance of Environment Variables
Environment variables provide information about the environment in which a program is running, allowing the program to operate flexibly. For example, the `PATH` variable defines a list of directories to search for executable commands, and the `HOME` variable stores the path to the user's home directory.
Key Roles of env
- View Environment Variables: Outputs all environment variables currently set in the shell.
- Set Execution Environment for a Command: Runs a specific command with temporary additions or changes to environment variables. This does not affect the existing shell environment.
- Script Debugging: Can be used to check the state of environment variables during script execution or to prevent a script from running in an unexpected environment.
Key env Command Options
The `env` command is primarily used without options or when setting variables and running a command. There are a few useful options available.
1. Basic Usage
2. Specific Environment Control Options
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`env` Executes the command.
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Usage Examples
Explore various examples of the `env` command to learn how to effectively manage environment variables and run commands in a specific environment.
Output all current environment variables
envChecks the list of all environment variables in the current logged-in shell.
Set a specific environment variable and run a command
env MY_TEST_VAR="hello" echo $MY_TEST_VARSets the environment variable `MY_TEST_VAR` to `hello` and then runs the `echo $MY_TEST_VAR` command. The variable disappears after the `echo` command finishes.
Run a program with a new environment variable
env DEBUG_MODE=true my_programSets the `DEBUG_MODE` environment variable to `true` and runs `my_program`. `my_program` can refer to this variable to operate in debug mode.
Run a command without the PATH variable (security test)
env -u PATH /bin/lsIgnores the existing `PATH` environment variable and runs the `/bin/ls` command. This shows that the `ls` command can be executed by specifying its direct path, even if it's not in `PATH`.
Run a script in an empty environment
env -i bash my_script.shIgnores all existing environment variables and runs the `my_script.sh` script in a completely clean environment. This is useful for testing if the script relies on unexpected environment variables.
Add a new path to PATH and run a command
env PATH=$PATH:/usr/local/python/bin python my_script.pyAdds a new path to the existing `PATH` and then runs the `python` command. This can be useful when you need to use a specific version of Python.