先说明一下:
如果第一个命令执行成功,与操作符 (&&)才会执行第二个命令
如果第一个命令执行失败,或操作符 (||)才会执行第二个命令
可以分析一下下面的命令的输出:
#!/bin/sh echo "" && echo "" || echo "" && echo "" || echo "" || echo "" && echo "" && echo "" || echo ""
下面是输出:
pengdl@debian:~/test/shell$ ./sh5.sh
分析:
echo "1" 执行成功,所以执行echo "2"
echo "2" 执行成功,不执行 echo "3"
既然没有执行echo "3",可以看做不存在 “ " || echo "3" ”,echo "2"后紧接“ && echo "4" ”
echo "4" 执行成功,不执行echo "5"
既然没有执行echo "5",可以看做不存在 “ " || echo "5" ”,echo "4"后紧接“ || echo "6" ”
echo "4" 执行成功,不执行echo "6"
既然没有执行echo "6",可以看做不存在 “ " || echo "6" ”,echo "4"后紧接“ && echo "7" ”
echo "7" 执行成功,执行echo "8"
echo "8" 执行成功,不执行echo "9"
所以最终变成了:
echo "" && echo "" && echo "" && echo "" && echo ""
改变一下,比如当前目录没存在目录p,不存在文件7
#!/bin/sh cd p && echo "" || echo "" && echo "" || echo "" || echo "" && ls "" && echo "" || echo ""
下面是输出:
pengdl@debian:~/test/shell$ ./sh6.sh
./sh6.sh: line : cd: p: No such file or directory ls: cannot access : No such file or directory
如果想改变执行逻辑,可以加优先操作符()
(Command_x1 &&Command_x2) || (Command_x3 && Command_x4)
在上面的伪代码中,如果Command_x1执行失败,Command_x2不会执行,但是Command_x3会继续执行, Command_x4会依赖于 Command_x3的退出状态。
(Command_x1 &&Command_x2)
它的执行成功与否: 如果Command_x1执行成功,执行Command_x2 ,如果Command_x2执行成功,那么整个()执行成功,如果Command_x2执行失败,整个()执行失败。如果Command_x2不执行,整个()的执行成功与否取决于Command_x1。
可以分析一下下面的例子:
例一
#!/bin/sh cd p && (echo "" || echo "") || (echo "" && echo "") && (ls || echo "" || (echo "" && echo ""))
下面是输出:
./sh8.sh: line : cd: p: No such file or directory ls: cannot access : No such file or directory
例二
#!/bin/sh cd p && (echo "" || echo "") || (echo "" && cd p) && (ls || echo "" || (echo "" && echo ""))
下面是输出:
./sh7.sh: line : cd: p: No such file or directory ./sh7.sh: line : cd: p: No such file or directory
例三:
#!/bin/bash if ls ./iii
then
echo ""
elif echo "" && ls ./i
then
echo ""
else
echo ""
fi
下面是输出:
pengdl@debian:~/test/shell$ ./sh8.sh
ls: cannot access ./iii: No such file or directory ls: cannot access ./i: No such file or directory