scl enable python27 bash activates a python virtual environment.

You can do this from within a bash script by simply sourcing the enable script of the virtual environment, of the SCL package, which is located at /opt/rh/python27/enable

Example:

#!/bin/bash
cd /var/www/python/scripts/
source /opt/rh/python27/enable
python runAllUpserts.py >/dev/null 2>&1

to get the script to act as you want it to, you need the scl command to modify the current bash session rather than spawn a new one.

you can do it this way

#!/usr/bin/env bash

# switch to GCC9 environment for the duration of the script
source scl_source enable devtoolset-9

gcc --version

Redhat proposes placing a file in /etc/profile.d, i.e. for python:

$ cat /etc/profile.d/enablepython33.sh
#!/bin/bash
source scl_source enable python33

As this works for the devtools under centos for me you could try this.

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.

This option sounds dangerous to me for root. I would think something like the following would be safer and more appropriate:

You can create a function that takes command line options. Think of this as an alias on steroids. Add the following to your .bashrc

python27() {
scl enable python27 “python $*”
}

Then test:

python27 –version
Python 2.7.5

This doesn’t help with your magic line in scripts, but will make it easier to call scripts:

[smccarty@keith ~]$ cat script.py
#!/usr/bin/env python27

import sys

print “Hello, World!”, sys.version

Call it normal and notice, the default installation of python is used:

[smccarty@keith ~]$ ./script.py
Hello, World! 2.6.6 (r266:84292, Sep 4 2013, 07:46:00)
[GCC 4.4.7 20120313 (Red Hat 4.4.7-3)]

Call it with our alias, and notice that Python 2.7 is used:

[smccarty@keith ~]$ python27 script.py
Hello, World! 2.7.5 (default, May 23 2013, 06:08:09)
[GCC 4.4.7 20120313 (Red Hat 4.4.7-3)]

An alternative of source /opt/rh/devtoolset-4/enable is

source scl_source enable devtoolset-4

The above shell script scl_source is more elegant than using a hard coded path (may be different on another machine). However scl_source does less because /opt/rh/devtoolset-4/enable uses scl_source and other stuff.

To use scl_source you may have to upgrade package scl-utils

yum update scl-utils  # old scl-utils versions miss scl_source

Quick copy-paste

echo 'source scl_source enable devtoolset-4' >> ~/.bashrc
    # Do not forget to change the version ↑

Source code for curious people

An example of scl_source source code:
https://gist.github.com/bkabrda/6435016

The scl_source installed on my Red Hat 7.1

#!/bin/bash

_scl_source_help="Usage: source scl_source <action> [<collection> ...]

Don't use this script outside of SCL scriptlets!

Options:
    -h, --help    display this help and exit"

if [ $# -eq 0 -o $1 = "-h" -o $1 = "--help" ]; then
    echo "$_scl_source_help"
    return 0
fi


if [ -z "$_recursion" ]; then
    _recursion="false"
fi
if [ -z "$_scl_scriptlet_name" ]; then
    # The only allowed action in the case of recursion is the same
    # as was the original
    _scl_scriptlet_name=$1
fi
shift 1

if [ -z "$_scl_dir" ]; then
    # No need to re-define the directory twice
    _scl_dir=/etc/scl/conf
    if [ ! -e $_scl_dir ]; then
        _scl_dir=/etc/scl/prefixes
    fi
fi

for arg in "$@"; do
    _scl_prefix_file=$_scl_dir/$arg
    _scl_prefix=`cat $_scl_prefix_file 2> /dev/null`
    if [ $? -ne 0 ]; then
        echo "Can't read $_scl_prefix_file, $arg is probably not installed."
        return 1
    fi

    # First check if the collection is already in the list
    # of collections to be enabled
    for scl in ${_scls[@]}; do
        if [ $arg == $scl ]; then
            continue 2
        fi
    done

    # Now check if the collection isn't already enabled
    /usr/bin/scl_enabled $arg > /dev/null 2> /dev/null
    if [ $? -ne 0 ]; then
        _scls+=($arg)
        _scl_prefixes+=($_scl_prefix)
    fi;
done

if [ $_recursion == "false" ]; then
    _i=0
    _recursion="true"
    while [ $_i -lt ${#_scls[@]} ]; do
        _scl_scriptlet_path="${_scl_prefixes[$_i]}/${_scls[$_i]}/${_scl_scriptlet_name}"
        source "$_scl_scriptlet_path"
        if [ $? -ne 0 ]; then
            echo "Can't source $_scl_scriptlet_name, skipping."
        else
            export X_SCLS="${_scls[$_i]} $X_SCLS"
        fi;
        _i=$(($_i+1))
    done
    _scls=()
    _scl_prefixes=()
    _scl_scriptlet_name=""
    _recursion="false"
fi