构造此简单的if语句时，为什么会出现错误“期望一元运算符”？

Question

I'm making a program, lab5, to test if the argument is a directory or a file. when I run the program with "bash lab5", I get "unary operator expected on line 3 and 5" What is the problem? Thanks!

#!/bin/bash

if test "$1" -d
then
    echo "This is a directory"
elif test "$1" -f
then 
    echo "This is a file"
else
    echo "This is neither a file or a directory"

fi

Answer 1

test does not have postfix operators, only prefix and infix operators. -f and -d are both prefix operators, meaning they come before the argument you want to test.

if test -d "$1"
then
    echo "This is a directory"
elif test -f "$1"
then 
    echo "This is a file"
else
    echo "This is neither a file or a directory"
fi

The exact error occurs because the shell sees something like test foo -d , and notes that test received exactly two arguments. With two arguments, it proceeds as follows:

1. If the first argument is ! , it just checks if the second argument is empty or not.
2. Otherwise, the first argument must be a unary operator, which foo is not.