Question

I have a file called x.sh that I want to execute. If I run:

x.sh

then I get:

x.sh: command not found

If I run:

./x.sh

then it runs correctly. Why do I have to type in ./ first?

Answer 1

Because the current directory is not into the PATH environment variable by default, and executables without a path qualification are searched only inside the directory specified by PATH. You can change this behavior by adding . to the end of PATH, but it’s not common practice, you’ll just get used to this UNIXism.

The idea behind this is that, if executables were searched first inside the current directory, a malicious user could put inside his home directory an executable named e.g. ls or grep or some other commonly used command, tricking the administrator to use it, maybe with superuser powers. On the other hand, this problem is not much felt if you put . at the end of PATH, since in that case the system directories are searched first.

But: our malicious user could still create his dangerous scripts named as common typos of often used commands, e.g. sl for ls (protip: bind it to Steam Locomotive and you won’t be tricked anyway :D).

So you see that it’s still better to be safe that, if you type an executable name without a path qualification, you are sure you’re running something from system directories (and thus supposedly safe).