假設定義了一個變數為,程式碼如下:
file=/dir1/dir2/dir3/my.file.txt
可以用${ }分別替換得到不同的值：
${file#*/}： 刪掉第一個 / 及其左邊的字串：dir1/dir2/dir3/my.file.txt 非貪婪匹配
${file##*/}： 刪掉最後一個 / 及其左邊的字串：my.file.txt 貪婪匹配
${file#*.}： 刪掉第一個 . 及其左邊的字串：file.txt
${file##*.}： 刪掉最後一個 . 及其左邊的字串：txt
${file%/*}： 刪掉最後一個 / 及其右邊的字串：/dir1/dir2/dir3
${file%%/*}： 刪掉第一個 / 及其右邊的字串：(空值)
${file%.*}： 刪掉最後一個 . 及其右邊的字串：/dir1/dir2/dir3/my.file
${file%%.*}： 刪掉第一個 . 及其右邊的字串：/dir1/dir2/dir3/my
記憶的方法為：
# 是 去掉左邊(鍵盤上#在 $ 的左邊)
% 是去掉右邊(鍵盤上% 在$ 的右邊)
單一符號是非貪婪匹配；兩個符號是貪婪匹配
${file:0:5}：提取最左邊的 5 個位元組：/dir1
${file:5:5}：提取第 5 個位元組右邊的連續5個位元組：/dir2
也可以對變數值裡的字串作替換：
${file/dir/path}：將第一個dir 替換為path：/path1/dir2/dir3/my.file.txt
${file//dir/path}：將全部dir 替換為 path：/path1/path2/path3/my.file.txt
[root@localhost ~]# p=123abc
[root@localhost ~]# echo ${p//[0-9]/} #將變數中的數字替換為空
abc
+———————————————————————-+
|Form Meaning
+———————————————————————-+
|${variable:?word} Complain if undefined or null
|${variable:-word} Use new value if undefined or null
|${variable:+word} Opposite of the above
|${variable:=word} Use new value if undefined or null, and redefine.
+———————————————————————-+
foo=${bar:-something}
echo $foo # something
echo $bar # no assignement to bar, bar is still empty
foo=${bar:=something}
echo $foo # something
echo $bar # something too, as there`s an assignement to bar
man bash ：
${parameter:-word}
Use Default Values. If parameter is unset or null, the expansion of word is substituted. Otherwise,
the value of parameter is substituted.
${parameter:=word}
Assign Default Values. If parameter is unset or null, the expansion of word is assigned to parameter.
The value of parameter is then substituted. Positional parameters and special parameters may not be
assigned to in this way.
${parameter:?word}
Display Error if Null or Unset. If parameter is null or unset, the expansion of word (or a message to
that effect if word is not present) is written to the standard error and the shell, if it is not inter-
active, exits. Otherwise, the value of parameter is substituted.
${parameter:+word}
Use Alternate Value. If parameter is null or unset, nothing is substituted, otherwise the expansion of
word is substituted.