Use Redefined command from PExpect

Question

How can I redefine a command in a bash script so that a python script called from the bash script will execute the redefined version when called via pexpect?

test.sh

#!/bin/bash
function scp(){
    echo "Skip SCP"
}
export -f scp
python test.py

test.py

import pexpect
scp = pexpect.spawn("scp")
scp.expect([pexpect.EOF,pexpect.TIMEOUT],timeout=1500)
print scp.before    

In this example I expect (and want) to see is:

Skip SCP

but what I actually see is:

usage: scp [-1246BCpqrv] [-c cipher] [-F ssh_config] [-i identity_file]
           [-l limit] [-o ssh_option] [-P port] [-S program]
           [[user@]host1:]file1 ... [[user@]host2:]file2

I can change anything about the shell script, but the python comes from a third party and is copied into many different projects, so changing it would be impractical.

Answer 1

Most likely spawn executes it's commands directly (via execv or something) or it uses a specific shell like /bin/sh. If you want it to use a customized environment, you'll have to specify that yourself. I don't know pexpect, but something like

spawn bash
expect your_prompt
send "function scp { echo 'skip scp'; }"
expect your_prompt
send scp
expect "skip scp"

Additionally, bash functions are not exported to child processes unless you export -f scp

---

Since you can't touch the pexpect part, the only thing you can change is scp. You will have to provide a program named "scp" that occurs earlier in the path than the regular scp

#!/bin/sh
PATH=/my/special/path:$PATH
cat > /my/special/path/scp <<END
#!/bin/sh
echo "no scp here!"
END
chmod 755 /my/special/path/scp

python test.py