如何在 Bash 中使用正负索引获取子字符串
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How to Get a Substring Using Positive and Negative Indexes in Bash
提问by Eli
What I want is pretty simple. Given a string 'this is my test string,' I want to return the substring from the 2nd position to the 2nd to last position. Something like:
substring 'this is my test string' 1,-1. I know I can get stuff from the beginning of the string using cut, but I'm not sure how I can calculate easily from the end of the string. Help?
我想要的很简单。给定一个字符串“这是我的测试字符串”，我想将子字符串从第二个位置返回到第二个到最后一个位置。类似的东西：
substring 'this is my test string' 1,-1。我知道我可以使用 cut 从字符串的开头获取内容，但我不确定如何从字符串的末尾轻松计算。帮助？
回答by Eli
Turns out I can do this with awk pretty easily as follows:
结果我可以很容易地用 awk 做到这一点，如下所示：
echo 'this is my test string' | awk '{ print substr( $0, 2, length($0)-2 ) }'
echo 'this is my test string' | awk '{ print substr( $0, 2, length($0)-2 ) }'
回答by AlG
Be cleaner in awk, python, perl, etc. but here's one way to do it:
在 awk、python、perl 等中更简洁，但这是一种方法：
#!/usr/bin/bash
msg="this is my test string"
start=2
len=$((${#msg} - ${start} - 2))
echo $len
echo ${msg:2:$len}
results in is is my test stri
结果是 is is my test stri
回答by bash-o-logist
You can do this with just pure bash
你可以用纯粹的 bash
$ string="i.am.a.stupid.fool.are.you?"
$ echo ${string: 2:$((${#string}-4))}
am.a.stupid.fool.are.yo
回答by l0b0
Look ma, no global variables or forks (except for the obvious printf) and thoroughly tested:
看 ma，没有全局变量或分叉（除了明显的printf）并经过彻底测试：
substring()
{
# Extract substring with positive or negative indexes
# @param : String
# @param : Start (default start of string)
# @param : Length (default until end of string)
local -i strlen="${#1}"
local -i start="${2-0}"
local -i length="${3-${#1}}"
if [[ "$start" -lt 0 ]]
then
let start+=$strlen
fi
if [[ "$length" -lt 0 ]]
then
let length+=$strlen
let length-=$start
fi
if [[ "$length" -lt 0 ]]
then
return
fi
printf %s "${1:$start:$length}"
}
