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• $* : Positional parameters. Separated by IFS. | $1c$2c$3
• $@ : Positional parameters. Separated by space. | $1 $2 $3
• “$@” : Positional parameters. Separated by space. Separated words. | “$1” “$2” “$3”
• $# : Number of positional parameters.
• $? : Exit status of the most recently executed foreground pipeline.
• $- : Current option flags as specified upon invocation, by the set builtin command, or those set by the shell itself (such as the -i option).
• $$ : Process ID of the shell. In a () subshell, it expands to the process ID of the invoking shell, not the subshell.
• $! : Process ID of the most recently executed background (asynchronous) command.
• $0 : Name of the shell or shell script.
• $_ : Last argument to the previous command, after expansion. (Or absolute shell pathname if first command).

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• Simple usage
  • $PARAMETER
  • ${PARAMETER}
• Indirection
  • ${!PARAMETER}
• Case modification
  • ${PARAMETER^}
  • ${PARAMETER^^}
  • ${PARAMETER,}
  • ${PARAMETER,,}
  • ${PARAMETER~}
  • ${PARAMETER~~}
• Variable name expansion
  • ${!PREFIX*}
  • ${!PREFIX@}
• Substring removal (also for filename manipulation!)
  • ${PARAMETER#PATTERN}
  • ${PARAMETER##PATTERN}
  • ${PARAMETER%PATTERN}
  • ${PARAMETER%%PATTERN}
• Search and replace
  • ${PARAMETER/PATTERN/STRING}
  • ${PARAMETER//PATTERN/STRING}
  • ${PARAMETER/PATTERN}
  • ${PARAMETER//PATTERN}
• String length
  • ${#PARAMETER}
• Substring expansion
  • ${PARAMETER:OFFSET}
  • ${PARAMETER:OFFSET:LENGTH}
• Use a default value
  • ${PARAMETER:-WORD}
  • ${PARAMETER-WORD}
• Assign a default value
  • ${PARAMETER:=WORD}
  • ${PARAMETER=WORD}
• Use an alternate value
  • ${PARAMETER:+WORD}
  • ${PARAMETER+WORD}
• Display error if null or unset
  • ${PARAMETER:?WORD}
  • ${PARAMETER?WORD}

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• !! : expands to the last command and all arguments
• !-3 : 3rd-to-last command and all arguments
• !^ : first argument of the last command in history
• !:2 : 2nd argument of the last command
• !$ : last argument of the last command
• !* : all arguments of the last command, but not the command itself
• !42 : expands to the 42nd command in the history list
• !foo : last command beginning with “foo”
• !?baz : last command containing “baz”
• ^foo^bar : last command with the first occurrence of “foo” replaced with “bar”
• !:gs/foo/bar : last command with all occurrences of “foo” replaced with “bar”
• <any_above>:p : prints command without executing