Question

I’m attempting to construct and execute a csh script from python.

The code I have produces what looks like a correct script, and os.system("my_script.csh") returns ‘0’, but the script doesn’t perform the task within it unless I go into it manually using vim and re-save it (changing nothing in the script manually – i don’t even enter ‘insert’ mode). What is it that re-saving in vim does that isn’t being done in my code, and is it possible to do it?

Here’s the relevant part of my code:

grabmeName = '%sgrabme%s.csh'%(dirNames['grabmes'],uniqID)
if not os.path.exists(grabmeName):
    open(grabmeName,'w').close()
    os.chmod(grabmeName,0777)
    with open(grabmeName,'a') as f:
        f.write("#!/bin/csh -f\n")
        f.write("echo 'hello'")
    os.system(grabmeName)

Answer 1

The main problem is that every line in the shell needs to end with a \n in order to be executed, even the last line. You can just add \n to the end of the "echo 'hello'" string. This is arguably a bug in csh, since bash and friends don’t have this problem, but if you want to use csh, you’ll have to accommodate it.

When you save a text file in vim, it adds a trailing newline to the file if there wasn’t one to begin with. You can verify this by saving a copy of the file beforehand and running diff to see what vim changes:

$ cat blah-grabme-12.csh 
#!/bin/csh -f
echo 'hello'$ cp blah-grabme-12.csh blah-grabme-12.csh.orig
$ vim blah-grabme-12.csh
┌────────────────────────────────────────────────────────────────────────────────┐
│#!/bin/csh -f                                                                   │
│echo 'hello'                                                                    │
│~                                                                               │
│~                                                                               │
│~                                                                               │
│~                                                                               │
│~                                                                               │
│~                                                                               │
│~                                                                               │
│~                                                                               │
│~                                                                               │
│~                                                                               │
│~                                                                               │
│~                                                                               │
│~                                                                               │
│~                                                                               │
│~                                                                               │
│~                                                                               │
│~                                                                               │
│~                                                                               │
│~                                                                               │
│~                                                                               │
│~                                                                               │
│:wq❚                                                                            │
└────────────────────────────────────────────────────────────────────────────────┘
$ diff -u blah-grabme-12.csh.orig blah-grabme-12.csh.orig
--- blah-grabme-12.csh.orig
+++ blah-grabme-12.csh
@@ -1,2 +1,2 @@
 #!/bin/csh -f
-echo 'hello'
\ No newline at end of file
+echo 'hello'

You can turn off this behaviour of vim in your ~/.vimrc if you’d like. See :help 'eol' in vim help.

Another potential problem is that system(filename) will only work if filename is a non-bare path—i.e., has a / in it—or if . is in the system $PATH.

Additionally, by using os.open() instead of open(), you can set the file permissions at file creation time. Here it might not make a big difference, but in many contexts, creating the file and then changing its permissions results in a security vulnerability. This stackoverflow question shows how to do that.

---

Putting it all together, you’d get something like this:

import os
import os.path

grabmeName = 'blah-grabme-12.csh'

with os.fdopen(os.open(grabmeName, os.O_WRONLY | os.O_CREAT, 0700), 'w') as f:
    f.write("#!/bin/csh -f\n")
    f.write("echo 'hello'\n")
os.system(os.path.abspath(grabmeName))