An echo operation includes some piece of text it should not include

Question

On a WSL Ubuntu 16.04 (Xenial) with composer I executed:

echo 2 * 3 > 5 is a valid inequality 

It resulted in a file by the name of 5, with the content:

2 composer-setup.php 3 is a valid inequality

I don't know why this echo includes the text composer-setup.php as well; it should have included everything from left and right besides 5 (the file name) as echo can have a single stream of text as its file name for redirection (no spaces allowed, as I understand, hence the 5 name).

Why is the composer-setup.php appears there between the 2 and 3?

Answer 1

You've executed a commandline that has two shell-special characters in it: * and >. You saw what > did; the * is a wildcard / globbing character that picked up every¹ file in your current directory. You have one file, named composer-setup.php in your current directory.

Quote your commandline; single-quotes work if there are no single-quotes in your text:

echo '2 * 3 > 5 is a valid inequality'

1: every file (or directory, socket, etc) that doesn't start with a period, unless you've set a shell option (such as dotglob) to specifically enable dot-file globbing.

Answer 2

There are two things happening here...

First is that the shell sees the * character and replaces it with a list of files in the current directory. This is known as "globbing"

In addition the > 5 is seen as a redirection command.

so

echo 2 * 3 > 5 is a valid inequality 

Is parsed as

echo 2 * 3 is a valid inequality > 5

This results in a file called "5" being created with the contents "2 list of current files 3 is a valid inequality".

To prevent this from happening we need to use quotes

echo '2 * 3 > 5 is a valid inequality'

The '...' prevents the shell from interpreting the special characters and displays the string you entered.