Command-line shells

Follow one line from the keyboard to the processes it starts. The shell's line editor — Readline in bash, ZLE in zsh, fish's own — handles keys, history search and Tab, calling the completion system for candidates. On Enter, the shell parses the line and runs its expansions: variables, command substitution, globs and, in the Bourne family, word splitting. Builtins such as `cd` or `export` run inside the shell itself; everything else becomes a child process, and pipes connect each child's standard output to the next one's standard input. When the child is itself a script, the kernel's shebang handling picks its interpreter, which starts non-interactive and reads its own, shorter list of startup files. PowerShell reshapes the middle of that path. Cmdlets and functions run in the same process and hand objects to each other; a native program such as `git` is still a child process exchanging text and returning an exit code, so one pipeline can mix both models. Configuration attaches to this path by mode: profile files for login shells, rc files for interactive ones, and in zsh a file that every instance reads. Options and emulation modes sit underneath all of it, changing how the parser and expander behave. That is why the same few lines can carry three meanings: ```sh #!/bin/sh # the shebang picks the interpreter: dash on Debian, bash on RHEL files="a.txt b.txt" for f in $files; do # sh and bash split this into two words printf '%s\n' "$f" done # typed at a zsh prompt, the loop runs once with one argument # adding [[ -n $files ]] works where /bin/sh is bash, fails under dash ``` Reading the snippet well means naming the interpreter first, then its expansion rules, then its extensions — the order in which a strong answer handles most questions in this hub.

This guide assumes bash 5.x, zsh 5.x, current fish and PowerShell 7.x. Several interview questions turn on machines that run something else: - **macOS** still ships bash 3.2, the last release under GPLv2, and has made zsh the default login shell since Catalina (10.15). Advice about "bash on a Mac" often describes features the system bash lacks. - **Debian and Ubuntu** point `/bin/sh` at dash rather than bash, a switch made long ago for startup speed; Red Hat-family systems still link it to bash. The same script can therefore pass on one family and fail on the other. - **PowerShell** exists as two products. Windows PowerShell 5.1 is bundled with Windows and runs on .NET Framework; PowerShell 6 began the cross-platform line on .NET Core, and 7 is its current form, installed separately as `pwsh`. Some Windows-only cmdlets did not make the move. - **fish 4.0** moved the implementation from C++ to Rust without redesigning the language users write. When an answer depends on version — an array feature, a cmdlet, a default option — say which one you mean.

Interviewers expect you to separate the pieces around a shell. The **terminal emulator** draws characters and forwards keystrokes; a **multiplexer** such as tmux keeps sessions alive across disconnects; the **shell** interprets what is typed. Prompt themes, plugin managers and frameworks such as oh-my-zsh are configuration layered on a shell, not shells of their own. For scripting, the usual comparison is with a general-purpose language. A shell suits gluing programs together, short automation and container entrypoints; once a script grows data structures, involved error handling or tests, Python or Go is usually cheaper to maintain. ShellCheck, a static analyser for sh and bash scripts, is the common answer to "how do you catch shell bugs before running them". Among interactive shells, bash is what most servers offer, zsh is the macOS default and popular for completion and plugins, and fish trades POSIX compatibility for defaults that need little configuration. Newer shells such as Nushell bring the structured-data side of the divide to Unix, passing tables rather than text. PowerShell on Linux is mostly chosen by teams that already automate Windows with it, and for fleet state it is weighed against configuration tools such as Ansible.