[Shell 基础] 字符串操作

1. 字符串长度
echo ${#str}

2. 字符串转为数字形式计算
echo $((str-1))

3. 截取指定长度字符串 (下标从 0 开始)
echo ${str:0:3}

4. 字符串比较
4-1.
if [ $str != "" ]; then
# if [ $str = "" ]; then
    echo $str
fi

4-2.
test "abc"  = "abc" ; echo $? # 0
test "abc" != "abc" ; echo $? # 1

4-3. 字符串长度的判断

[ "" ]; echo $? #1

[ "a" ]; echo $? # 0
test -z "";  echo $? # 0
test -z "a"; echo $? # 1
test -n "";  echo $? # 1
test -n "a"; echo $? # 0

5. 字符串连结
str1="a"
str2="b"
str=$str1$str2"c"
echo $str

6. 设置默认值
变量 AAA 未设置时, 使用默认值, 设置时, 使用设置的值
#!bin/sh
#AAA="world"
BBB=${AAA:-"hello"}
echo "AAA="$AAA  # 空
echo "BBB="$BBB  # hello


AAA="world"
BBB=${AAA:-"hello"}
echo "AAA="$AAA  # world
echo "BBB="$BBB  # world