CODEWITHSUNDEEP
pythonbashpexpect
Vainmonde De Courtenay
Vainmonde De Courtenay

Reputation: 376

Running multiple Bash commands interactively from Python

I have just come across pexpect and have been figuring out how to use it to automate various practices I would otherwise have to fill in manually in a command shell.

Here's an example script:

import pexpect, sys

child = pexpect.spawn("bash", timeout=60)
child.logfile = sys.stdout
child.sendline("cd /workspace/my_notebooks/code_files")
child.expect('#')
child.sendline('ls')
child.expect('#')
child.sendline('git add .')
child.expect('#')
child.sendline('git commit')
child.expect('#')
child.sendline('git push origin main')
child.expect('Username .*:')
child.sendline(<my_github_username>)
child.expect('Password .*:')
child.sendline(<my_github_password>)
child.expect('#')
child.expect(pexpect.EOF)

(I know these particular tasks do not necessarily require pexpect, just trying to understand its best practices.)

Now, the above works. It cds to my local repo folder, lists the files there, stages my commits, and pushes to Github with authentication, all the while providing real-time output to the Python stdout. But I have two areas I'd like to improve:

Firstly, .expect('#') between every line I would run in Bash (that doesn't require interactivity) is a little tedious. (And I'm not sure whether / why it always seems to work, whatever was the output in stdout - although so far it does.) Ideally I could just clump them into one multiline string and dispense with all those expects. Isn't there a more natural way to automate parts of the script that could be e.g., a multiline string with Bash commands separated by ';' or '&&' or '||'?

Secondly, if you run a script like the above you'll see it times out after 60 seconds sharp, then yields a TimeoutError in Python. Although - assuming the job fits within 60 seconds - it gets done, I would prefer something which (1) doesn't take unnecessarily long, (2) doesn't risk cutting off a >60 second process midway, (3) doesn't end the whole thing giving me an error in Python. Can we instead have it come to a natural end, i.e., when the shell processes are finished, that's when it stops running in Python too? (If (2) and (3) can be addressed, I could probably just set an enormous timeout value - not sure if there is better practice though.)

What's the best way of rewriting the code above? I grouped these two issues in one question because my guess is there is a generally better way of using pexpect, which could solve both problems (and probably others I don't even know I have yet!), and in general I'd invite being shown the best way of doing this kind of task.

Upvotes: 0

Views: 609

Answers (1)

Barmar
Barmar

Reputation: 780724

You don't need to wait for # between each command. You can just send all the commands and ignore the shell prompts. The shell buffers all the inputs.

You only need to wait for the username and password prompts, and then the final # after the last command.

You also need to send an exit command at the end, otherwise you won't get EOF.

import pexpect, sys

child = pexpect.spawn("bash", timeout=60)
child.logfile = sys.stdout
child.sendline("cd /workspace/my_notebooks/code_files")
child.sendline('ls')
child.sendline('git add .')
child.sendline('git commit')
child.sendline('git push origin main')
child.expect('Username .*:')
child.sendline(<my_github_username>)
child.expect('Password .*:')
child.sendline(<my_github_password>)
child.expect('#')
child.sendline('exit')
child.expect(pexpect.EOF)

If you're running into the 60 second timeout, you can use timeout=None to disable this. See pexpect timeout with large block of data from child

You could also combine multiple commands in a single line:

import pexpect, sys

child = pexpect.spawn("bash", timeout=60)
child.logfile = sys.stdout
child.sendline("cd /workspace/my_notebooks/code_files && ls && git add . && git commit && git push origin main')
child.expect('Username .*:')
child.sendline(<my_github_username>)
child.expect('Password .*:')
child.sendline(<my_github_password>)
child.expect('#')
child.sendline('exit')
child.expect(pexpect.EOF)

Using && between the commands ensures that it stops if any of them fails.

In general I wouldn't recommend using pexpect for this at all. Make a shell script that does everything you want, and run the script with a single subprocess.Popen() call.

Upvotes: 2

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