How to use scl command as a script shebang? - python-2.7

Question

If I want to run a specific command (with arguments) under Software Collections, I can use this command:
scl enable python27 "ls /tmp"
However, if I try to make a shell script that has a similar command as its shebang line, I get errors:
$ cat myscript
#!/usr/bin/scl enable python27 "ls /tmp"
echo hello
$ ./myscript
Unable to open /etc/scl/prefixes/"ls!
What am I doing wrong?

Answer 1

You should try using -- instead of surrounding your command with quotes.
scl enable python27 -- ls /tmp
I was able to make a python script that uses the rh-python35 collection with this shebang:
#!/usr/bin/scl enable rh-python35 -- python
import sys
print(sys.version)

Answer 2

The parsing of arguments in the she-bang command is not really defined. From man execve:
The semantics of the optional-arg argument of an interpreter script vary across implementations. On Linux, the entire string following the interpreter name is passed as a single argument to the interpreter, and this string can include white space. However, behavior differs on some other systems. Some systems use the first white space to terminate optional-arg. On some systems, an interpreter script can have multiple arguments, and white spaces in optional-arg are used to delimit the arguments.
No matter what, argument splitting based on quote sis not supported. So when you write:
#!/usr/bin/scl enable python27 "ls /tmp"
It's very possible that what gets invoked is (using bash notation):
'/usr/bin/scl' 'enable' 'python27' '"ls' '/tmp"'
This is probably why it tries to open the "ls file at /etc/scl/prefixes/"ls
But it is just as likely that the shebang evaluates to:
'/usr/bin/scl' 'enable python27 "ls /tmp"'
And that would fail since it wont be able to find a command named enable python27 "ls /tmp" for scl to execute.
There's a few workarounds you can use.
You can call your script via scl:
$ cat myscript
#!/bin/bash
echo hello
$ scl enable python27 ./myscript
hello
You can also use the heredoc notation, but it might lead to subtle issues. I personally avoid this:
$ cat ./myscript
#!/bin/bash
scl enable python27 -- <<EOF
echo hi
echo \$X_SCLS
EOF
$ bash -x myscript
+ scl enable python27 --
hi
python27
You can see one of the gotcha's already: I had to write \$X_SCLS to access the environment variable instead of just $X_SCL.
Edit: Another option is two have two scripts. One that has the actual code, and the second that simply does scl enable python27 $FIRST_SCRIPT. Then you wont have to remember to enter scl ... manually.

Answer 3

The software collections documentation may also be helpful. In particular you can try
cat myscript.sh | scl enable python27 -
As well as permanently enabling a software collection
source scl_source enable python27
./myscript.sh