Why equal to operator does not work if it is not s

问题:

I tried the following script

#!/bin/bash
var1=\"Test 1\" 
var2=\"Test 2\"
if [ \"$var1\"=\"$var2\" ] 
  then 
    echo \"Equal\" 
  else 
    echo \"Not equal\"
fi

It gave me Equal. Although it should have printed Not equal

Only when I inserted space around = it worked as intended

if [ \"$var1\" = \"$var2\" ] 

and printed Not equal

Why is it so? Why \"$var1\"=\"$var2\" is not same as \"$var1\" = \"$var2\"?

Moreover, when I wrote if [ \"$var1\"= \"$var2\" ], it gave

line 4: [: Test 1=: unary operator expected

What does it it mean? How come its expecting unary operator?

回答1:

test (or [ expr ]) is a builtin function. Like all functions in bash, you pass it\'s arguments as whitespace separated words.

As the man page for bash builtins states: \"Each operator and operand must be a separate argument.\"

It\'s just the way bash and most other Unix shells work.

Variable assignment is different.

In bash a variable assignment has the syntax: name=[value]. You cannot put unquoted spaces around the = because bash would not interpret this as the assignment you intend. bash treats most lists of words as a command with parameters.

E.g.

# call the command or function \'abc\' with \'=def\' as argument
abc =def

# call \'def\' with the variable \'abc\' set to the empty string
abc= def

# call \'ghi\' with \'abc\' set to \'def\'
abc=def ghi

# set \'abc\' to \'def ghi\'
abc=\"def ghi\"

回答2:

When the shell reads

if [ \"$var1\" = \"$var2\" ]

it invokes the command [ with 4 arguments. Whether [ is a builtin or an external command is irrelevant, but it may help to understand that it may be the external command /bin/[. The second argument is the literal \'=\' and the fourth is \']\'. However, when the shell reads

if [ \"$var1\"= \"$var2\" ]

[ only gets 3 arguments: the expansion of $var1 with \'=\' appended, the expansion of $var2, and \']\'. When it gets only 3 arguments, it expects the last argument to be \']\' and the first argument to be a unary operator.

回答3:

To add to the existing explanation, \"$var1\"=\"$var2\" is just a single non-empty string, and thus always evaluates as true in a conditional.

[ \"$var1\"=\"$var2\" ] && echo true

The above command will always print out true (even if var1 and var2 be empty).

回答4:

In bash the best is to use [[ ]]:

x=\"test\"
y=\"test\"
if [[ \"${x}\" = \"${y}\" ]]; then
    echo \"Equals\"
else
    echo \"No equals\"
fi