Why do you need ./ (dot-slash) before executable or script name to run it in bash?

Question

When running scripts in bash, I have to write ./ in the beginning:

$ ./manage.py syncdb 

If I don't, I get an error message:

$ manage.py syncdb -bash: manage.py: command not found 

What is the reason for this? I thought . is an alias for current folder, and therefore these two calls should be equivalent.

I also don't understand why I don't need ./ when running applications, such as:

user:/home/user$ cd /usr/bin user:/usr/bin$ git 

(which runs without ./)

Answer 1

Because on Unix, usually, the current directory is not in $PATH.

When you type a command the shell looks up a list of directories, as specified by the PATH variable. The current directory is not in that list.

The reason for not having the current directory on that list is security.

Let's say you're root and go into another user's directory and type sl instead of ls. If the current directory is in PATH, the shell will try to execute the sl program in that directory (since there is no other sl program). That sl program might be malicious.

It works with ./ because POSIX specifies that a command name that contain a / will be used as a filename directly, suppressing a search in $PATH. You could have used full path for the exact same effect, but ./ is shorter and easier to write.

EDIT

That sl part was just an example. The directories in PATH are searched sequentially and when a match is made that program is executed. So, depending on how PATH looks, typing a normal command may or may not be enough to run the program in the current directory.

Answer 2

When bash interprets the command line, it looks for commands in locations described in the environment variable $PATH. To see it type:

echo $PATH 

You will have some paths separated by colons. As you will see the current path . is usually not in $PATH. So Bash cannot find your command if it is in the current directory. You can change it by having:

PATH=$PATH:. 

This line adds the current directory in $PATH so you can do:

manage.py syncdb 

It is not recommended as it has security issue, plus you can have weird behaviours, as . varies upon the directory you are in :)

Avoid:

PATH=.:$PATH 

As you can “mask” some standard command and open the door to security breach :)

Just my two cents.