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
Answers:
subprocess‘s rules for handling the command argument are actually a bit complex.
Generally speaking, to run external commands, you should use shell=False and pass the arguments as a sequence. Use shell=True only if you need to use shell built-in commands or specific shell syntax; using shell=True correctly is platform-specific as detailed below.
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)
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
subprocess‘s rules for handling the command argument are actually a bit complex.
Generally speaking, to run external commands, you should use shell=False and pass the arguments as a sequence. Use shell=True only if you need to use shell built-in commands or specific shell syntax; using shell=True correctly is platform-specific as detailed below.
From the docs:
argsshould be a sequence of program arguments or else a single string. By default, the program to execute is the first item inargsifargsis a sequence. Ifargsis a string, the interpretation is platform-dependent and described below. See theshellandexecutablearguments for additional differences from the default behavior. Unless otherwise stated, it is recommended to passargsas a sequence…. Ifshellis True, it is recommended to passargsas a string rather than as a sequence.
With shell=False:
On Unix, if
argsis 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
argsis 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 underlyingCreateProcess()operates on strings.
With shell=True:
On Unix with
shell=True, the shell defaults to/bin/sh. Ifargsis 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, theCOMSPECenvironment variable specifies the default shell. The only time you need to specifyshell=Trueon Windows is when the command you wish to execute is built into the shell (e.g.dirorcopy). You do not needshell=Trueto run a batch file or console-based executable.
(all emphasis mine)