字符串
字符串
字符串是 shell 编程中最常用最有用的数据类型，单引号里的任何字符都会原样输出，单引号字符串中的变量是无效的；双引号中则允许引入变量：
# 使用双引号拼接
greeting="hello, "$your_name" !"
greeting_1="hello, ${your_name} !"
# 使用单引号拼接
greeting_2='hello, '$your_name' !'
greeting_3='hello, ${your_name} !' # hello, ${your_name} !
字符串常见的操作如下：
string="string"
# 获取字符串长度
echo ${#string}
# 提取子字符串
${string:1:4}
# 查找子字符串
echo `expr index "$string" io`
引号
您的 Bash Shell 可以理解具有特殊含义的特殊字符。例如，$var 用于扩展变量值。Bash 扩展变量和通配符，例如：
echo "$PATH"
echo "$PS1"
echo /etc/*.conf
但是，有时您不希望使用变量或通配符。例如，不要打印 $PATH 的值，而只是在屏幕上将 $PATH 作为单词打印。您可以通过将特殊字符的含义用单引号引起来来启用或禁用。这对于在编写 Shell 脚本时抑制警告和错误消息也很有用。
echo "Path is $PATH" ## $PATH will be expanded
echo 'I want to print $PATH' ## PATH will not be expanded
引号分为三种：
| Quote type | Name | Meaning | Example (type at shell prompt) |
|---|---|---|---|
| " | The double quote | The double quote ( “quote” ) protects everything enclosed between two double quote marks except $, ‘, " and .Use the double quotes when you want only variables and command substitution. _ Variable - Yes _ Wildcards - No * Command substitution - yes | The double quotes allowes to print the value of $SHELL variable, disables the meaning of wildcards, and finally allows command substitution. echo "$SHELL"echo "/etc/*.conf"echo "Today is $(date)" |
| ' | The single quote | The single quote ( ‘quote’ ) protects everything enclosed between two single quote marks. It is used to turn off the special meaning of all characters. _ Variable - No _ Wildcards - No * Command substitution - No | The single quotes prevents displaying variable $SHELL value, disabled the meaning of wildcards /etc/*.conf, and finally command substitution ($date) itself. echo '$SHELL'echo '/etc/*.conf'echo 'Today is $(date)' |
| \ | The Backslash | Use backslash to change the special meaning of the characters or to escape special characters within the text such as quotation marks. | You can use \ before dollar sign to tell the shell to have no special meaning. Disable the meaning of the next character in $PATH (i.e. do not display value of $PATH variable): echo "Path is \$PATH"echo "Path is $PATH" |
\ 可以用来进行换行衔接：
echo "A monkey-tailed boy named Goku is found by an old martial \
>arts expert who raises him as his grandson. One day Goku meets a \
>girl named Bulma and together they go on a quest to retrieve the seven Dragon Balls"
# Purpose: clean /tmp/$domain ?
check_temp_clean() {
[ "$SERVER_MODE" = "daemon" ] || return 1
[ "$SERVER_MODE" = "init" ] && return 0
# note use of the backslash character to continue command on next line
[ "$SERVER_MODE" = "clean" \
-a -e /usr/local/etc/nixcraft/lighttpd/disk_cache.init ] && return 0
return 1
}