Shell 变量的截取

 

1. 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

2. 变量的转化

   1. 替换

${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