Expression after last specific character

Question

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

Can anyone explain how the "${foo##*/}" expression works? I understand it will return the string after the last forward slash (i.e. jfds). However, I have no idea how it does 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  ]]