invocation/

README

Command Invocation

A Command consists of a program name optionally followed by space separated arguments, for example:

ls
man ls

Manual pages, accessed via the man command, are a core feature of operating systems following the Unix tradition.

Commands inherit an environment, which consists of key-value paired variables. A parent process may customize the environment each child receives, or may pass its own environment along without modification. In a shell, environment variables may be accessed using dollar-sign notation, such as:

echo "$SHELL"

It's important to quote variables in order to avoid shell injection attacks and undefined behavior that can result from special characters and whitespace in the variable values (shells may re-interpret values after variable substitution has occurred).

Commands can be invoked with a custom environment by passing key-value pairs before the command name, such as:

# add SOMEVAR to the existing environment and display that environment
SOMEVAR=someval env

# use the env command as a command runner; in this case it'll just run
# env again, but with an empty environment
env -i env

If a variable needs to be in the environment of several commands, it can be more convenient to "export" it:

export SOMEVAR=someval
env

After exporting a variable, all subsequent commands invoked by the shell process will contain that variable in their environment. Conventionally, all environment variable names are fully upper-case. As with file and command names, you should always treat variable names as being case-sensitive.

Example

Invocation

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All material is licensed under the Apache License Version 2.0, January 2004.