Linux bash - 最后一个特定字符后的表达式
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bash - expression after last specific character
提问by toop
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)?
Canyone 解释了 " ${foo##*/}" 表达式是如何工作的，因为我知道它会在最后一个正斜杠（即 jfds）之后返回字符串，但我不知道它是如何做到的（或者这种类型的表达式被称为什么）？
采纳答案by Peter.O
It is one of several shell features, generically called shell expansion. This particular expansionis called parameterexpansion*.
它是多个 shell 功能之一，通常称为shell 扩展。这种特殊的扩展称为参数扩展*。
You can think of this particular shell expansion form as a left-truncatestring function. You must use the curly braces as shown (that is not optional)..
您可以将这种特殊的 shell 扩展形式视为左截断字符串函数。您必须使用如图所示的花括号（这不是可选的）。
When you use only one #, it means left-truncate only the firstoccurrence of the pattern which follows (up to the closing }. When you use two ##, it means left-truncate allconsecutive pattern-matches. The result of var="a/b/c"; echo ${var#*/}is b/c... echo ${var##*/}returns c.
当您只使用 one 时#，这意味着仅左截断随后出现的模式的第一次出现（直到结束}。当您使用 two 时##，这意味着左截断所有连续的模式匹配。结果var="a/b/c"; echo ${var#*/}是b/c...echo ${var##*/}返回 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).
有一个互补的右截断。它使用%而不是#...（我“记得”这是因为#就像一个bash注释；总是在左边）。
The *is treated as a bash wildcard expansion.
将*被视为一个bash通配符扩展。
Here is a list of all shell expansions, presented in precedence order.
这是所有 shell 扩展的列表，按优先顺序显示。
The order of expansions is:
展开顺序为：
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 ]]
