shell一些技巧和知识点
欢迎进入Linux社区论坛,与200万技术人员互动交流 >>进入 shell一些技巧和知识点 ; 表示的是一个命令执行完, 忽略其返回值, 继续执行下一行 表示只有前面为true才继续执行后面的命令, 否则忽略, 可以理解为短路执行 || 表示子前面为false才继续执行后面
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shell一些技巧和知识点
; 表示的是一个命令执行完, 忽略其返回值, 继续执行下一行
&& 表示只有前面为true才继续执行后面的命令, 否则忽略, 可以理解为短路执行
|| 表示子前面为false才继续执行后面的命令, 这个比以前看到的解释更容易理解
凡是要输出字符串的, 最好用引号括起来, 否则容易被shell理解为命令, 这一点很容易疏忽
{}是一种命令块的写法
:可以看成append操作
export用来将一个变量升级为环境变量
str=
这行代码表示将str这个变量设置为null
${str:-+=expr} 这些操作忘记了, 还需要巩固一下
source会将script看成当前shell进程的一部分, 而不是生成一个sub shell进程来执行脚本, source用来加载公共的函数, 可以理解为import
exec用来结束当前shell进程, 并重新生成一个新的进程来执行脚本
()用来将多个命令放到一个sub shell进程中去执行, 可以理解为将表达式变成函数
{} 理解成匿名函数吧, 一般在awk中常见
$()和``是一回事, 不过前者辨识度更高, 建议常用, 后者容易与''混淆, 另外一个前者嵌套起来更简单, 不过前者移植性较差
${}除了用来界定变量之外, 还有一个功能实现字符串变量的substring操作, #表示去掉左边的, %表示去掉右边的, 单数表示最小匹配, 双数表示最大匹配, 可以通过键盘上#和%相对于$的位置来记忆, linux上的命令参数大部分记忆起来很伤脑筋:(
${}还可以用来设置默认值, 这块更复杂, 不记也罢
${#str}用来计算变量的字符长度
()的另一个作用是用来定义一个数组, 而针对数组的访问则利用的是[], 注意数组的下标base是0, 另外@用来表示全部, 可以理解为all
$(())只针对算数和逻辑运算, 里面的变量可以不用带上$前缀, 当然带上也不为错
shell中的0表示true, 非零表示false
(())用来直接进行运算, 会让我们的代码更简洁, 比如变量不用加$前缀
$0表示的是脚本路径, $1…n代表的是脚本路径之后的参数
函数可以理解为脚本中的脚本, 不过函数中$0表示的仍然是脚本路径, 而不是函数名
$10不是表示第十个参数而是第一个参数后面带一个0
一般#跟数量关系比较大, 比如${#str}表示变量长度, 而$#表示参数个数, 测试是否有传参数 [ $# = 0 ]

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