Python OSError: [Errno 2]

Question

I have the following code that is attempting to start each of the "commands" below in Linux. The module attempts to keep each of the 2 commands running if either should crash for whatever reason.

#!/usr/bin/env python
import subprocess
commands = [ ["screen -dmS RealmD top"], ["screen -DmS RealmD top -d 5"] ]
programs = [ subprocess.Popen(c) for c in commands ]
while True:
 for i in range(len(programs)):
 if programs[i].returncode is None:
 continue # still running
 else:
 # restart this one
 programs[i]= subprocess.Popen(commands[i])
 time.sleep(1.0)

Upon executing the code the following exception is thrown:

Traceback (most recent call last):
 File "./marp.py", line 82, in <module>
 programs = [ subprocess.Popen(c) for c in commands ]
 File "/usr/lib/python2.6/subprocess.py", line 595, in __init__
 errread, errwrite)
 File "/usr/lib/python2.6/subprocess.py", line 1092, in _execute_child
 raise child_exception
OSError: [Errno 2] No such file or directory

I think I'm missing something obvious, can anyone see what's wrong with the code above?

• jfs: you should use programs[i].poll() instead of programs[i].returncode.
• jfs: related: subprocess.call using string vs using list

Answer 1

Use ["screen", "-dmS", "RealmD", "top"] instead of ["screen -dmS RealmD top"].

Maybe also use the complete path to screen.

• Colin Burnett: Docs say a string or a sequence can be used.
• Caedis: commands = [ ["screen", "-dmS", "RealmD", "top"], ["screen", "-DmS", "RealmD", "top", "-d", "5"] ] Worked perfectly!
• sth: @colin: In this case a sequence was used and it needs to contain the parameters separately. Probably a simple string (without the []) would also work.
• S.Lott: Either 1 string, or 1 parameter "word" per element of a list. If you pass a sequence, you should have done the exact parsing that a shell normally does. If you pass a single string it's parsed exactly the way the shell would have parsed it.
• jfs: @umpirsky: from the docs: "On Unix, if args is a string, the string is interpreted as the name or path of the program to execute. However, this can only be done _if not passing arguments to the program_."

Answer 2

Only guess is that it can't find screen. Try /usr/bin/screen or whatever which screen gives you.

• Brian Moeskau: Yep, this was my issue. Command was not in system path as expected.

Answer 3

The problem is that your command should be split. subprocces requires that the cmd is a list, not a string. It shouldn't be:

subprocess.call('''awk 'BEGIN {FS="\t";OFS="\n"} {a[1ドル]=a [1ドル] OFS 2ドル FS 3ドル FS 4ドル} END
{for (i in a) {print i a[i]}}' 2_lcsorted.txt > 2_locus_2.txt''') 

That won't work. If you feed subprocess a string, it assumes that that is the path to the command you want to execute. The command needs to be a list. Check out http://www.gossamer-threads.com/lists/python/python/724330. Also, because you're using file redirection, you should use subprocess.call(cmd, shell=True). You can also use shlex.

• WoJ: subprocess.call("executable -parameter 1 -hello", shell=True) works (assuming it is in the PATH of the shell)
• WoJ: It depends on the call. If you do not parse input then it is as safe as a shell command.

Answer 4

I got same error when i wrote like this :-

subprocess.Popen("ls" ,shell = False , stdout = subprocess.PIPE ,stderr = subprocess.PIPE)

And problem is solved when i made shell=True .It will work

subprocess.Popen("ls" ,shell = False , stdout = subprocess.PIPE ,stderr = subprocess.PIPE, shell=True)

Answer 5

commands = [ "screen -dmS RealmD top", "screen -DmS RealmD top -d 5" ]
programs = [ subprocess.Popen(c.split()) for c in commands ]

• Tanky Woo: Use shlex.split() instead of string.split()

Answer 6

Just in case.. I also got stuck with this error and the issue was that my files were in DOS instead of UNIX so at :

 return subprocess.call(lst_exp)

where lst_exp is a list of args, one of them was "not found" because it was in DOS instead of UNIX but error thrown was the same :

File "/var/www/run_verifier.py", line 59, in main
return subprocess.call(lst_exp)
File "/usr/lib/python2.7/subprocess.py", line 522, in call
return Popen(*popenargs, **kwargs).wait()
File "/usr/lib/python2.7/subprocess.py", line 710, in __init__
errread, errwrite)
File "/usr/lib/python2.7/subprocess.py", line 1335, in _execute_child
raise child_exception
OSError: [Errno 2] No such file or directory