====== Make A Script Executable ======

Normally, to run a script on Linux you would have to call it with the name of the application, for example with a Python script ''myscript.py'' you would type something like:

<code lang=shell-session>
$ python myscript.py
</code>

... or, if it was a shell script named ''myscript.sh'':

<code lang=shell-session>
$ bash myscript.sh
</code>

You can remove the need to explicitly call the application which runs the script if you have set the correct ''#!'' [[https://en.wikipedia.org/wiki/Shebang_(Unix)|hash-bang]] entry as the first line of the script and then set the script file to be executable.

By default our script file will not be executable. Check by using ''ls -l'':

<code lang=shell-session>
$ ls -l myscript.py
-rw------- 1 n1234 mygroup       47091 Nov  4 12:49 myscript.py
$
</code>

Notice the lack of ''x'' entries in the permissions columns. We can add execute with the ''chmod +x'':

<code lang=shell-session>
$ chmod +x myscript.sh
$ ls -l myscript.py
-rwx------ 1 n1234 mygroup       47091 Nov  4 12:49 myscript.py
$
</code>

Note that the script now shows ''x'' in the owner permission column. The script, as long as it has a valid ''#!/bin'' hash-bang first line should now run by simply calling the file itself, e.g.:

<code lang=shell-session>
$ ./myscript.py
</code>

<WRAP tip round>
===== Tip - Don't use hardcoded hash-bang paths =====

Most hash-bang paths specify the absolute path and filename of the application which runs the script, e.g:

<code lang=bash>
#!/bin/bash
</code>

... or ...

<code lang=bash>
#!/usr/bin/python3
</code>

In most circumstances this is okay, but on an HPC system where the version of Bash, Python, R or other runtime applications //may very well differ// it is often easier to use the following convention:

<code lang=bash>
#!/usr/bin/env python3
</code>

The ''/usr/bin/env'' command, instead of directly calling python3, will search all of the available ''$PATH'' directories for python3. This means that regardless of which Python 3 [[started:module_basics|module]] you decide to load, it will always call the current version.

You can use the same trick for Perl, R, Ruby and other script runtimes which you would normally explicitly set via the hash-bang line.
</WRAP>

----

[[:faq:index|Back to FAQ]]