linux - Unix 测试是否存在或文件

Question

[ -f /tmp/myfile.txt ] && echo "File exists" || echo "No such file"

这是如何运作的？特别是 && 和 || 的评估。

Answer 1

&& 和 || 是控制运算符，它们根据先前命令的退出状态确定是否应执行以下命令。来自 man (1) bash:

The control operators && and || denote AND lists and OR lists, respectively. An AND list has the form

  command1 && command2

command2 is executed if, and only if, command1 returns an exit status of zero.

An OR list has the form

  command1 || command2

command2 is executed if and only if command1 returns a non-zero exit status. The return status of AND and OR lists is the exit status of the last command executed in the list.

[ 或 test，如果命令通过测试，则返回 零 退出状态。来自帮助测试:

test: test [expr] Exits with a status of 0 (true) or 1 (false) depending on the evaluation of EXPR.

所以当你这样做的时候

[ -f /tmp/myfile.txt ] && echo foo || echo bar

你真正想说的是(请仔细阅读)如果文件存在，回显 foo。如果文件不存在或echo foo命令失败，回显栏。

这是这些控制运算符与 if...then 命令之间的细微但关键的区别。