Environment

Shell initialization files

• /etc/profile is systemwide bash profile
• /etc/bashrc has bash-specific configurations (so does zshrc)
• ~/bash_profile user specific config
• ~/bashrc read after system wide bashrc, aliases and stuff

Variables

• Global/Environment varaibles are available in all shells, display with env command.
• Local/Shell variables are available in the current shell, display with set command.
  - set -e sets a bash option
• MYVAR="Hello" defines a local variable in the current shell
• export MYVAR adds it to environment variables which allows child processes to have access, using the bash export builtin

Shell Expansion

A command is first tokenized, then one of 8 expansions is performed on each token, followed by quote removal.

• brace expansion generates a set of strings (first expansion)
  - sp{el, il, al}l => spell spill spall (prefixes and postfixes to each option)
• tilde expansion expands to the $HOME path
• parameter expansion with $ replaces an expression with its value
  • for ${PARAMETER}, the value of "PARAMETER" is substituted
  • braces are required when PARAMETER is follwed by a char, not part of the var name: "plural: ${WORD}s"
• command substitution substitutes the output of a command with $(command) or backticks
• arithmetic expansion treats expression as double quotes, and after expansions, evaluates numerically: $(( EXPRESSION ))
  • $[ EXPRESSION ] performs calculation, no expansion?, no tests
• lastly, performs word splitting on expansions, then file name expansion with glob patterns