What is a binary operator

Question

I made a code in bash:

#!/bin/bash

echo "Digite o arquivo: "
read ARQUIVO
PROCURA=$(find /home/gabriel -name $ARQUIVO)
test $PROCURA -e & echo "O arquivo '$ARQUIVO' não foi encontrado"

And when running it appears the following error message mentioning binary operators:

prompt$./testa-arquivos.sh 

Digite o arquivo: 
teste
O aruqivo 'teste' não foi encontrado
./testa-arquivos.sh: linha 6: test: Script/teste: esperado operador binário

How can I resolve this error, and I would also like to understand what binary operators are and what they are for.

Answer 1

Binary operator is one that gets two operands. And this can be a pain in Bash.

Something I usually do a lot is to check if one number is larger than or equal to another. For this, I use the test with the operator gt:

if [ "$numero" -gt 2 ]; then
    ...
fi

Where can the main bottlenecks with binary operators arise in Bash? Well, in addition to the syntax with behavior well out of the pattern (from those who come from a more behaved world like C, Java or Pascal), we have such of the variable expansions.

What are these expansions? Well, you noticed I put quotes around $numero? I did it with a single goal: if it has no value in $numero, place the empty string.

Imagine a scenario where $numero has no value; when doing variable expansion, my code would look like this:

if [ "" -gt 2 ]; then
    ...
fi

Now imagine that I had forgotten the quotation marks. My code would be expanded to the following:

if [  -gt 2 ]; then
    ...
fi

Note that in this second case, there is no operator on the left side of -gt, which leads to an error of comparison!

The other point I spoke about was the non-standard syntax. If you are not careful, a short-circuit of commands may end up turning into the call of a routine in parallel or opening a pipeline.

For example, the short circuit operator AND && indicates that I only execute the command on the right if the command on the left returns true (in the shell world, output code 0). The short circuit operator OR || indicates that I only execute the command on the right if the command on the left returns false (in the shell world, output code other than 0).

So let’s go to the parallel processing operators and pipeline. The operator & indicates that the command that proceeds must be executed in parallel; note that a simple typing error can transform the AND && the operator. The operator | will create a pipeline between the command on the left and the command on the right; note that it is easy to type the OR || and fall into the pipe.

Note that in your code, the command test is running in another thread as it is followed by &.

For more readings, see Shell Script Swiss Army Knife by Aurelio Verde

Answer 2

Answer your question in a simpler way.

Change the line:

test $PROCURA -e & echo "O aruqivo '$ARQUIVO' não foi encontrado"

Down the stretch :

if [  -e $PROCURA ]; then
echo 'O arquivo foi encontrado.'
else
echo 'O arquivo não foi encontrado'
fi

Well now let’s talk about why the error message on the line, in your test line the -e (binary operator) came after the variable, it should come before, but still your line would not work the way it was, because you would need a structure that validates the test. The test just tests, it has no action, you need the if to take the test result and make it in a condition of true or false.

Now comes the long explanation, but first let’s talk test command. you wondered why we are using the [ and not writing test?

Well, they are the same thing. Actually, there are 3 test commands, a bash native, /usr/bin/test and /usr/bin/[. But there are differences between them related to POSIX. It is quite complex to explain these differences, but for now if your script gives problem with [ you will have to use it so double brackets [[.

Binary operators are those you determine at test time, such as -e (exists), -f (filing cabinet) , -d (directory) and etc. see man page bash to see all.

Now goes an extra tip on logical operators:

Use -a for logical "and" and -o for logical "or", perhaps because of POSIX parameters the operators && and || not interpreted by bash. But you can still use that double bracket option [[.

I hope that’s clear to you.