bash - expression after last specific character

Question

foo="/sdf/here/jfds"
bar="${foo##*/}"

Canyone explain how the "${foo##*/}" expression works because I understand it will return the string after the last forward slash (ie jfds) but I have no idea how it does it (or what this type of expression is called)?

Answer 1

It is one of several shell features, generically called shell expansion. This particular expansion is called parameter expansion*.

You can think of this particular shell expansion form as a left-truncate string function. You must use the curly braces as shown (that is not optional)..

When you use only one #, it means left-truncate only the first occurrence of the pattern which follows (up to the closing }. When you use two ##, it means left-truncate all consecutive pattern-matches. The result of var="a/b/c"; echo ${var#*/} is b/c... echo ${var##*/} returns c.

There is a complementary right-truncate. It uses % instead of the #... (I "remember" which is which because # is like a bash comment; always on the left).

The * is treated as a bash wildcard expansion.

Here is a list of all shell expansions, presented in precedence order.

The order of expansions is:

1. brace expansion ... prefix{-,\,}postfix             # prefix-postfix prefix,postfix
                    .. {oct,hex,dec,bin}               # oct hex dec bin
                     . {a..b}{1..2}                    # a1 a2 b1 b2
                     . {1..04}                         # 01 02 03 04
                     . {01..4}                         # 01 02 03 04
                     . {1..9..2}                       # 1 3 5 7 9
                     . \$\'\\x{0..7}{{0..9},{A..F}}\'  # $'\x00' .. $'\x7F'     

2. tilde expansion .... ~           # $HOME
                    ... ~axiom      # $(dirname "$HOME")/axiom  
                    ... ~fred       # $(dirname "$HOME")/fred
                     .. ~+          # $PWD     (current working directory)
                     .. ~-          # $OLDPWD  (previous working directory. If OLDPWD is unset,
                                                        ~- is not expanded. ie. It stays as-is,
                                                          regardless of the state of nullglob.)
                                    # Expansion for Directories in Stack. ie. 
                                    # The list printed by 'dirs' when invoked without options 
                      . ~+N         #    Nth directory in 'dirs' list (from LHS)
                      . ~-N         #    Nth directory in 'dirs' list (from RHS)

3. parameter expansion .... ${VAR/b/-dd-}  
                        ... ${TEST_MODE:-0}
                         .. ${str: -3:2}  # note space after :
                          . ${#string}

4. (processed left-to-right) 
     variable expansion 
     arithmetic expansion
     command substitution

▶5. word splitting          # based on $IFS (Internal Field Seperator)

▷6. pathname expansion
      according to options such as:   
      nullglob, GLOBIGNORE, ...and more

# Note: ===============
▶ 5. word splitting     ↰ 
▷ 6. pathname expansion ↰  
# =====================  ↳  are not performed on words between  [[  and  ]]