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变量算术运算
使用${#VAR} 语法将计算一个变量当中字符的数量
[bob in ~] echo $SHELL
/bin/bash
[bob in ~] echo ${#SHELL}
9
[bob in ~] ARRAY=(one two three)
[bob in ~] echo ${#ARRAY}
3
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变量的转化
-
替换
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${VAR:-WORD} 如果 VAR 没有定义或者是空值，由WORD 的值替换；否则输出 VAR 的值：
[bob in ~] echo ${TEST:-test}
test
[bob in ~] echo $TEST
[bob in ~] export TEST=a_string
[bob in ~] echo ${TEST:-test}
a_string
[bob in ~] echo ${TEST2:-$TEST}
a_string
如果 (-) 使用等号(=)替换，后者的值会被赋值到前者上:
[bob in ~] echo $TEST2
[bob in ~] echo ${TEST2:=$TEST}
a_string
以下语法测试一个变量是否存在。如果没有设置，WORD 的扩展打印到标准输出且非交互shell退出。
[bob in ~]echo ${TEST2:?WORD}
(与${VAR:-WORD}的区别，${VAR:?WORD}是一个错误，不是一个正常的输出，而前者回正常输出
和${VAR:=WORD})的区别，${VAR:?WORD}不会把word的值赋给var)
证明：
[bob in ~] cat vartest.sh
#!/bin/bash
# This script tests whether a variable is set. If not,
# it exits printing a message.
echo ${TESTVAR:?"There's so much I still wanted to do..."}
echo "TESTVAR is set, we can proceed."
[bob in testdir] ./vartest.sh
./vartest.sh: line 6: TESTVAR: There's so much I still wanted to do...
[bob in testdir] export TESTVAR=present
[bob in testdir] ./vartest.sh
present
TESTVAR is set, we can proceed.
3.字符串切片
要从一个变量之中，删除(注意，这里是删除而不是截取)等于 OFFSET 数量的字符，使用这样的语法：
${VAR:OFFSET:LENGTH}
LENGTH 参数定义了在偏移点offset之后第一个字符开始需要保留多少字符。如果 LENGTH 省略，就是用剩余的变量的内容。
[bob in ~] export STRING="thisisaverylongname"
[bob in ~] echo ${STRING:4}
isaverylongname
[bob in ~] echo ${STRING:6:5}
avery
${VAR#WORD} and ${VAR##WORD}
注：这里是这样定义的：ARRAY=(one two one three one four),是数组
而这样定义的ARRAY="one two one three one four"是一个字符串，数组会对每一个元素操作。
[bob in ~] echo ${ARRAY[*]}
one two one three one four
[bob in ~] echo ${ARRAY[*]#one}
two three four
[bob in ~] echo ${ARRAY[*]#t}
one wo one hree one four
[bob in ~] echo ${ARRAY[*]#t*}
one wo one hree one four
表示对每个元素，从左开始数，直到匹配到t*的最短的为止，截除掉。
[bob in ~] echo ${ARRAY[*]##t*}
one one one four
表示对每个元素，从左开始数，直到匹配到t*的最长的为止，截除掉。
[bob in ~] echo $STRING
thisisaverylongname
%与#作用类似，但是从变量的右边开始匹配
[bob in ~] echo ${STRING%name}
thisisaverylong
4.替换：
${VAR/PATTERN/STRING}
or
${VAR//PATTERN/STRING}
语法。第一种形式仅仅替换第一个匹配的项目，第二个用 STRING 替换所有匹配 PATTERN 的项目。
[bob in ~] echo ${STRING/name/string}
thisisaverylongstring