检查文件是否可读并且是否存在于一个 if 条件中: if [[ -r -f "/file.png";]] [英] Checking if a file is readable and exists in one if condition: if [[ -r -f "/file.png" ]]

问题描述

我正在编写一个 if 语句，它通过执行以下操作检查文件是否可读和存在:

I was writing an if statement which checked if a file is readable and exists by doing the following:

if [[ -r "$upFN" && -f "$upFN" ]]; then
....
fi

然后我想，你可以把它弄小一点，可能是这样的:

Then I thought, surly you can make this smaller, something maybe like this:

if [[ -r -f "$upFN" ]]; then
....
fi

但这不起作用，它返回错误:

But this doesn't work, it returns errors:

./ftp.sh: line 72: syntax error in conditional expression
./ftp.sh: line 72: syntax error near `"$upFN"'
./ftp.sh: line 72: `if [[ -r -f "$upFN" ]]; then'

推荐答案

AFAICT，没有办法进一步组合它们.作为可移植性说明，[[ expr ]] 的可移植性不如 [ expr ] 或 test expr.C 风格的 && 和 || 仅包含在 bash 中，因此您可能需要考虑使用 -a 的 POSIX 语法and 和 -o 用于 or.就个人而言，我更喜欢使用 test expr 因为它非常明确.许多 shell(包括 bash)都包含一个内置函数，因此您不必担心进程创建开销.

AFAICT, there is no way to combine them further. As a portability note, [[ expr ]] is less portable than [ expr ] or test expr. The C-style && and || are only included in bash so you might want to consider using the POSIX syntax of -a for and and -o for or. Personally, I prefer using test expr since it is very explicit. Many shells (bash included) include a builtin for it so you do not have to worry about process creation overhead.

无论如何，我会将您的测试重写为:

In any case, I would rewrite your test as:

if test -r "$upFN" -a -f "$upFN"
then
  ...
fi

该语法适用于传统的 Bourne shell、Korn shell 和 Bash.您也可以使用可移植的 [ 语法.

That syntax will work in traditional Bourne shell, Korn shell, and Bash. You can use the [ syntax portably just as well.

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