Bash/Shell Programming – Binary Operator Expected
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Now that our scripts are getting a little more complicated, I want to point out some common mistakes that you might run into. To do this, create the following script called trouble. Be sure to enter it exactly as written. When you run this script, it should output the line "Number equals 1" because, well, number equals 1. If you don't get the expected output, check your typing; you made a mistake.
Number does not equal 1. As you can see, bash displayed an error message when we ran the script. You probably think that by removing the "1" on line 3 it created a syntax error on line 3, but it didn't.
Let's look at the error message again:. We can see that. Remember that " [ " is an abbreviation for the test shell builtin. From this we can determine that the error is occurring on line 5 not line 3.
First, let me say there is nothing wrong with line 3. You will sometimes want to set a variable's value to nothing. You can confirm the validity of this by trying it on the command line:.
To understand this error, we have to see what the shell sees. Remember that the shell spends a lot of its life expanding text. It also explains the rest of the error message we received.
What the shell is trying conditional binary operator expected bash tell us is that there is only one item and there should be a unary operator like "! This brings up an important thing to remember when you are writing your scripts. Consider what happens if a conditional binary operator expected bash is set to equal nothing. Here we have another case of a mistake in one line causing a problem later in the script. What happens is the shell keeps looking for the closing quotation mark to tell it where the end of the string is, but runs into the end of the file before it finds it.
These errors can be a real pain to find in a long script. This is one reason you should test your scripts frequently when you are writing them so there is less new code to test. I also find that text editors with syntax highlighting make these kinds of bugs easier to find.
Finding bugs in your programs can sometimes be very difficult and frustrating. Here are a couple of techniques that you will find useful:. Isolate blocks of code by "commenting them out. Frequently, you will do this to a block of code to see if a particular problem goes away. Conditional binary operator expected bash doing this, you can isolate which part of a program is causing or not causing a problem.
By commenting out the else clause and running the script, we could show that the problem was not in the else clause even though the error message suggested that it was. Use echo commands to verify your assumptions. As you gain experience tracking down bugs, you will discover that bugs are often not where you first expect to find them.
A common problem will be that you will make a false assumption about the performance of your program. You will see a problem develop at a certain point in your program and assume that the problem is there. This conditional binary operator expected bash often incorrect, as we have seen. To combat this, you should place echo commands in your code while you are debugging, to produce messages that confirm the program is doing what is expected.
There are two kinds of messages that you should insert. The first type simply conditional binary operator expected bash that you have conditional binary operator expected bash a certain point in the program.
We saw this conditional binary operator expected bash our earlier discussion on stubbing. It is useful to know that program flow is happening the way we expect. The second type displays the value of a variable or variables conditional binary operator expected bash in a calculation or test. You will often find that a portion of your program will fail because something that you assumed was correct earlier in your program is, in fact, incorrect and is causing your program to fail later on.
It is possible to have bash show you what it is doing when you run your script. To do this, add a " -x " to the conditional binary operator expected bash line of your script, like this:. Now, when you run your script, bash will display each line with expansions performed as it executes it.
This technique is called tracing. Here is what it looks like:. Alternately, you can use the set command within your script to turn tracing on and off. Previous Contents Top Next. Verbatim copying and distribution of this entire article is permitted in any medium, provided conditional binary operator expected bash copyright notice is preserved. Previous Contents Next Stay Out Of Trouble Now that our scripts are getting a little more complicated, I want to point out some common mistakes that you might run into.
Empty Variables Edit the script to change line 3 from: This time you should get the following: Number does not equal 1 As you can see, bash displayed an error message when we ran the script. Let's look at the error message again: You can confirm the validity of this by trying it on the command line: So what's wrong with line 5? To fix this problem, change line 5 to read: Missing Quotes Edit line 6 to remove the trailing quote from the end of the line: You should get this: Isolating Problems Finding bugs in your programs can sometimes be very difficult and frustrating.
Here are a couple of techniques that you will find useful: For example, when we were looking for our missing quotation we could have done this: Watching Your Script Run It is possible to have bash show you what it is doing when you run your script. To do this, add a " -x " to the first line of your script, like this: Here is what it looks like: