Working on the command line

Variables, and the word export does all the work

A plain assignment — NAME=value, with no spaces around the equals sign — creates a shell variable, visible to this shell and to nothing else. export NAME turns it into an environment variable, which means it is copied into every process the shell starts afterwards. That is the whole distinction, and it explains the classic failure: a variable set but not exported is invisible to the script that needed it.

set with no arguments lists shell variables and functions; env lists the environment. env is also a command that runs a program with a modified environment, which is why env VAR=x command is the idiom for setting something for one command only. unset removes a variable. Changes made in a shell die with it, so anything that must survive goes into a startup file — ~/.bashrc for an interactive shell, ~/.bash_profile for a login shell, /etc/profile and /etc/profile.d/ for everyone.

Which program actually runs

When you type a name, bash checks in order: aliases, shell functions, builtins, then each directory in PATH from left to right. type NAME reports which of those it found and is therefore the honest answer; which searches PATH only and will happily tell you about /bin/time while the shell runs its own builtin. Adding a directory to PATH is PATH=$PATH:/new/dir — losing the $PATH is how a shell ends up unable to find ls.

history recalls previous commands, stored in ~/.bash_history and controlled by HISTSIZE (how many are kept in memory) and HISTFILESIZE (how many in the file). !! repeats the last command and !n repeats command n. uname reports the kernel — uname -r for the release, uname -a for everything — and is the command a question means when it asks how to identify a running kernel version.

Quoting, and the manual

Double quotes suppress globbing and word splitting but still expand $variables and backticks. Single quotes suppress everything, so the text is literally what you typed. A backslash escapes exactly the next character. Getting this wrong is how a script passes a literal $HOME to a program.

man PAGE reads the manual; man -k or apropos searches the short descriptions when you do not know the page name. Manual sections matter: 1 is user commands, 5 is file formats, 8 is administration, so man 5 passwd is the file format and man 1 passwd is the command. uname, echo, pwd and exec round out the vocabulary — exec replaces the shell with the program rather than starting a child, which is why a script ending in exec leaves no extra process behind.