subprocess.call using string vs using list

Question

I am trying to use rsync with subprocess.call. Oddly, it works if I pass subprocess.call a string, but it won't work with a list (ala, Python's doc).

calling sp.call with a string:

In [23]: sp.call("rsync -av content/ writings_raw/", shell=True) sending incremental file list  sent 6236 bytes  received 22 bytes  12516.00 bytes/sec total size is 324710  speedup is 51.89 Out[23]: 0 

calling sp.call with a list:

In [24]: sp.call(["rsync", "-av", "content/", "writings_raw/"], shell=True) rsync  version 3.0.9  protocol version 30 Copyright (C) 1996-2011 by Andrew Tridgell, Wayne Davison, and others. Web site: http://rsync.samba.org/ Capabilities:     64-bit files, 64-bit inums, 32-bit timestamps, 64-bit long ints,     socketpairs, hardlinks, symlinks, IPv6, batchfiles, inplace,     append, ACLs, xattrs, iconv, symtimes  rsync comes with ABSOLUTELY NO WARRANTY.  This is free software, and you are welcome to redistribute it under certain conditions.  See the GNU General Public Licence for details.  rsync is a file transfer program capable of efficient remote update via a fast differencing algorithm.  Usage: rsync [OPTION]... SRC [SRC]... DEST   or   rsync [OPTION]... SRC [SRC]... [USER@]HOST:DEST   or   rsync [OPTION]... SRC [SRC]... [USER@]HOST::DEST   or   rsync [OPTION]... SRC [SRC]... rsync://[USER@]HOST[:PORT]/DEST   or   rsync [OPTION]... [USER@]HOST:SRC [DEST]   or   rsync [OPTION]... [USER@]HOST::SRC [DEST]   or   rsync [OPTION]... rsync://[USER@]HOST[:PORT]/SRC [DEST] The ':' usages connect via remote shell, while '::' & 'rsync://' usages connect to an rsync daemon, and require SRC or DEST to start with a module name.  Options  -v, --verbose               increase verbosity  -q, --quiet                 suppress non-error messages      --no-motd               suppress daemon-mode MOTD (see manpage caveat) ... snipped....                              repeated: --filter='- .rsync-filter'      --exclude=PATTERN       exclude files matching PATTERN      --blocking-io           use blocking I/O for the remote shell  -4, --ipv4                  prefer IPv4  -6, --ipv6                  prefer IPv6      --version               print version number (-h) --help                  show this help (-h is --help only if used alone) ...snipped ... rsync error: syntax or usage error (code 1) at main.c(1438) [client=3.0.9] Out[24]: 1 

What is wrong with how I use the list? How would you fix it? I need the list, because I would like to use variables. Of course I could use:

  sp.call("rsync -av "+Orig+" "+Dest, shell=True)     

But I would like to understand how subprocess understands lists vs. strings.

setting shell=False and a list:

In [36]: sp.call(['rsync', '-av', ORIG, DEST], shell=False) sending incremental file list  sent 6253 bytes  received 23 bytes  12552.00 bytes/sec total size is 324710  speedup is 51.74 Out[36]: 0 

setting shell=False and a string

In [38]: sp.call("rsync -av"+" "+ORIG+" "+DEST, shell=False) --------------------------------------------------------------------------- OSError                                   Traceback (most recent call last) <ipython-input-38-0d366d3ef8ce> in <module>() ----> 1 sp.call("rsync -av"+" "+ORIG+" "+DEST, shell=False)  /usr/lib/python2.7/subprocess.pyc in call(*popenargs, **kwargs)     491     retcode = call(["ls", "-l"])     492     """ --> 493     return Popen(*popenargs, **kwargs).wait()     494      495   /usr/lib/python2.7/subprocess.pyc in __init__(self, args, bufsize, executable, stdin, stdout, stderr, preexec_fn, close_fds, shell, cwd, env, universal_newlines, startupinfo, creationflags)     677                             p2cread, p2cwrite,     678                             c2pread, c2pwrite, --> 679                             errread, errwrite)     680      681         if mswindows:  /usr/lib/python2.7/subprocess.pyc in _execute_child(self, args, executable, preexec_fn, close_fds, cwd, env, universal_newlines, startupinfo, creationflags, shell, p2cread, p2cwrite, c2pread, c2pwrite, errread, errwrite)    1257                     if fd is not None:    1258                         os.close(fd) -> 1259                 raise child_exception    1260     1261   OSError: [Errno 2] No such file or directory 

Answer 1

subprocess's rules for handling the command argument are actually a bit complex.

From the docs:

args should be a sequence of program arguments or else a single string. By default, the program to execute is the first item in args if args is a sequence. If args is a string, the interpretation is platform-dependent and described below. See the shell and executable arguments for additional differences from the default behavior. Unless otherwise stated, it is recommended to pass args as a sequence.... If shell is True, it is recommended to pass args as a string rather than as a sequence.

With shell=False:

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.

On Windows, if args is a sequence, it will be converted to a string in a manner described in Converting an argument sequence to a string on Windows. This is because the underlying CreateProcess() operates on strings.

With shell=True:

On Unix with shell=True, the shell defaults to /bin/sh. If args is a string, the string specifies the command to execute through the shell. This means that the string must be formatted exactly as it would be when typed at the shell prompt. This includes, for example, quoting or backslash escaping filenames with spaces in them. If args is a sequence, the first item specifies the command string, and any additional items will be treated as additional arguments to the shell itself.

On Windows with shell=True, the COMSPEC environment variable specifies the default shell. The only time you need to specify shell=True on Windows is when the command you wish to execute is built into the shell (e.g. dir or copy). You do not need shell=True to run a batch file or console-based executable.

(all emphasis mine)