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2023-09-08 |
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This page will document Unix history, and POSIX knowledge for working with [[Linux]], [[BSD]], [[macOS]], and others.
POSIX is a documented set of features and behaviors that all "UNIX" systems are supposed to comply with in order for various software/tools to be cross-compatible.
A shell is just the way you interact with an operating system, but usually when we talk about a "shell", it's in the context of command-line processing shells like [[bash]], csh, tcsh, zsh, fish, and others. POSIX.
Bash as well as other shells will be some level POSIX compliant, either by default, via flags, configs, or when invoked with the name (argv 0) of sh.
Simply put, a user has a single "login" shell instance, which is the shell they logged in with. This includes when you [[ssh]] into another machine.
When a POSIX login shell is executed, it will source the system-wide /etc/profile config file, followed by the logging-in user's ~/.profile.
(Note that Bash will read more files, documented on the [[Bash]] page.)
An interactive shell is one that allows a user to interact with it. A non-interactive shell is any shell that is not user facing, such as a sub-shell in a script, or invoked by a program.
Some shells will execute scripts for an interactive but not a login shell. This is shell dependent, and is not a POSIX standard. (Bash's files are documented on the [[Bash]] page.)