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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).

Source

• Simple usage
  • $PARAMETER
  • ${PARAMETER}
• Indirection
  • ${!PARAMETER} : Value of the variable whose name is PARAMETER.
• Case modification
  • ${PARAMETER^} : First upper.
  • ${PARAMETER^^} : Whole upper.
  • ${PARAMETER,} : First lower.
  • ${PARAMETER,,} : Whole lower.
  • ${PARAMETER~} : Switch first.
  • ${PARAMETER~~} : Switch all.
• Variable name expansion
  • ${!PREFIX*} : List of all variables and arrays names beginning with PREFIX.
  • ${!PREFIX@} : Idem.
• Substring removal (also for filename manipulation!)
  • ${PARAMETER#PATTERN} : Strip left (short match).
  • ${PARAMETER##PATTERN} : Strip left (long match).
  • ${PARAMETER%PATTERN} : Strip right (short match).
  • ${PARAMETER%%PATTERN} : Strip right (long match).
• Search and replace
  • ${PARAMETER/PATTERN/STRING} : Replace (fisrt occurence).
  • ${PARAMETER//PATTERN/STRING} : Replace (all occurences).
  • ${PARAMETER/PATTERN} : Search (first occurence).
  • ${PARAMETER//PATTERN} Search (all occurrences).
• String length
  • ${#PARAMETER}
• Substring expansion
  • ${PARAMETER:OFFSET}
  • ${PARAMETER:OFFSET:LENGTH}
• Use a default value
  • ${PARAMETER:-WORD} : WORD when PARAMETER is unset or empty.
  • ${PARAMETER-WORD} : WORD when PARAMETER is unset.
• Assign a default value
  • ${PARAMETER:=WORD} : Expand AND assign WORD when PARAMETER is unset or empty.
  • ${PARAMETER=WORD} : Expand AND assign WORD when PARAMETER is unset.
• Use an alternate value
  • ${PARAMETER:+WORD} : Nothing if PARAMETER is unset or empty, WORD else.
  • ${PARAMETER+WORD} : Nothing if PARAMETER is unset, WORD else.
• Display error if null or unset
  • ${PARAMETER:?WORD} : Like :- AND sets non-null exit code and $?.
  • ${PARAMETER?WORD} : Like - AND sets non-null exit code and $?.

• dirname=${fullpath%/*}
• filename=${fullpath##*/}
• extension=${filename##*.}

• ~% FILE="example.tar.gz"
  ~% echo "${FILE%%.*}"
  example
  ~% echo "${FILE%.*}"
  example.tar
  ~% echo "${FILE#*.}"
  tar.gz
  ~% echo "${FILE##*.}"
  gz

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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

• Diriger plusieurs lignes vers un fichier :

  /bin/cat <<EOM >$FILE
  text1
  text2
  text3
  text4
  EOM

func_name () { cmd1; cmd2 }

On retrouve cette syntax au coeur de la fameuse fork bomb bask :(){ :|:& };: où : est le nom de la fonction. L'utilisation du pipe et des : est a priori seulement à pour obfusquer un peu plus la syntaxe (:) et la raccourcir (|). f(){f&;f&;}f fait quaisment la même chose.