Local Commands

Plumbum exposes a special singleton object named local, which represents your local machine and serves as a factory for command objects:

>>> from plumbum import local
>>>
>>> ls = local["ls"]
>>> ls
<LocalCommand C:\Program Files\Git\bin\ls.exe>
>>> notepad = local["c:\\windows\\notepad.exe"]
>>> notepad
<LocalCommand c:\windows\notepad.exe>

If you don’t specify a full path, the program is searched for in your system’s PATH (and if no match is found, a CommandNotFound exception is raised). Otherwise, the full path is used as given. Once you have a Command object, you can execute it like a normal function:

>>> ls()
'README.rst\nplumbum\nsetup.py\ntests\ntodo.txt\n'
>>> ls("-a")
'.\n..\n.git\n.gitignore\n.project\n.pydevproject\nREADME.rst\n[...]'

If you use the .get() method instead of [], you can include fallbacks to try if the first command does not exist on the machine. This can be used to get one of several equivalent commands, or it can be used to check for common locations of a command if not in the path. For example:

pandoc = local.get('pandoc',
                   '~/AppData/Local/Pandoc/pandoc.exe',
                   '/Program Files/Pandoc/pandoc.exe',
                   '/Program Files (x86)/Pandoc/pandoc.exe')

An exception is still raised if none of the commands are found. Unlike [] access, an exception will be raised if the executable does not exist.

New in version 1.6: The .get method

With just a touch of magic, you can import commands from the mock module cmd, like so:

>>> from plumbum.cmd import grep, cat
>>> cat
<LocalCommand C:\Program Files\Git\bin\cat.exe>

Note

There’s no real module named plumbum.cmd; it’s a dynamically-created “module”, injected into sys.modules to enable the use of from plumbum.cmd import foo. As of version 1.1, you can actually import plumbum.cmd, for consistency, but it’s not recommended.

It is important to stress that from plumbum.cmd import foo translates to local["foo"] behind the scenes.

If underscores (_) appear in the name, and the name cannot be found in the path as-is, the underscores will be replaced by hyphens (-) and the name will be looked up again. This allows you to import apt_get for apt-get.

Pipelining

In order to form pipelines and other chains, we must first learn to bind arguments to commands. As you’ve seen, invoking a command runs the program; by using square brackets (__getitem__), we can create bound commands:

>>> ls["-l"]
BoundCommand(<LocalCommand C:\Program Files\Git\bin\ls.exe>, ('-l',))
>>> grep["-v", ".py"]
BoundCommand(<LocalCommand C:\Program Files\Git\bin\grep.exe>, ('-v', '.py'))

You can think of bound commands as commands that “remember” their arguments. Creating a bound command does not run the program; in order to run it, you’ll need to call (invoke) it, like so: ls["-l"]() (in fact, ls["-l"]() is equivalent to ls("-l")).

Now that we can bind arguments to commands, forming pipelines is easy and straight-forwards, using | (bitwise-or):

>>> chain = ls["-l"] | grep[".py"]
>>> print chain
C:\Program Files\Git\bin\ls.exe -l | C:\Program Files\Git\bin\grep.exe .py
>>>
>>> chain()
'-rw-r--r--    1 sebulba  Administ        0 Apr 27 11:54 setup.py\n'

Input/Output Redirection

We can also use redirection into files (or any object that exposes a real fileno()). If a string is given, it is assumed to be a file name, and a file with that name is opened for you. In this example, we’re reading from stdin into grep world, and redirecting the output to a file named tmp.txt:

>>> import sys
>>> ((grep["world"] < sys.stdin) > "tmp.txt")()
hello
hello world
what has the world become?
foo                                    # Ctrl+D pressed
''

Note

Parenthesis are required here! grep["world"] < sys.stdin > "tmp.txt" would result in False...

Right after foo, Ctrl+D was pressed, which caused grep to finish. The empty string at the end is the command’s stdout (and it’s empty because it actually went to a file). Lo and behold, the file was created:

>>> cat("tmp.txt")
'hello world\nwhat has the world become?\n'

If you need to send input into a program (through its stdin), instead of writing the data to a file and redirecting this file into stdin, you can use the shortcut << (shift-left):

>>> (cat << "hello world\nfoo\nbar\spam" | grep["oo"]) ()
'foo\n'

Exit Codes

If the command we’re running fails (returns a non-zero exit code), we’ll get an exception:

>>> cat("non/existing.file")
Traceback (most recent call last):
  [...]
ProcessExecutionError: Command line: ['C:\\Program Files\\Git\\bin\\cat.exe', 'non/existing.file']
Exit code: 1
Stderr:  | "C:/Program Files/Git/bin/cat.exe": non/existing.file: No such file or directory

In order to avoid such exceptions, or when a different exit code is expected, just pass retcode = xxx as a keyword argument. If retcode is None, no exception checking is performed (any exit code is accepted); otherwise, the exit code is expected to match the one you passed:

>>> cat("non/existing.file", retcode = None)
''
>>> cat("non/existing.file", retcode = 17)
Traceback (most recent call last):
  [...]
ProcessExecutionError: Command line: ['C:\\Program Files\\Git\\bin\\cat.exe', 'non/existing.file']
Exit code: 1
Stderr:  | "C:/Program Files/Git/bin/cat.exe": non/existing.file: No such file or directory

Note

If you wish to accept several valid exit codes, retcode may be a tuple or a list. For instance, grep("foo", "myfile.txt", retcode = (0, 2))

If you need to have both the output/error and the exit code (using exceptions would provide either but not both), you can use the run method, which will provide all of them

>>>  cat["non/existing.file"].run(retcode=None)
(1, u'', u'/bin/cat: non/existing.file: No such file or directory\n')

If you need the value of the exit code, there are two ways to do it. You can call .run(retcode=None) (or any other valid retcode value) on a command, you will get a tuple (retcode, stdout, stderr) (see Run and Popen. If you just need the recode, or want to check the retcode, there are two special objects that can be applied to your command to run it and get or test the retcode. For example:

>>> cat["non/existing.file"] & RETCODE
1
>>> cat["non/existing.file"] & TF
False
>>> cat["non/existing.file"] & TF(1)
True

Note

If you want to run these commands in the foreground (see Background and Foreground), you can give FG=True to TF or RETCODE. For instance, cat["non/existing.file"] & TF(1,FG=True)

New in version 1.5: The TF and RETCODE modifiers

Run and Popen

Notice that calling commands (or chained-commands) only returns their stdout. In order to get hold of the exit code or stderr, you’ll need to use the run() method, which returns a 3-tuple of the exit code, stdout, and stderr:

>>> ls.run("-a")
(0, '.\n..\n.git\n.gitignore\n.project\n.pydevproject\nREADME.rst\nplumbum\[...]', '')

You can also pass retcode as a keyword argument to run in the same way discussed above.

And, if you want to want to execute commands “in the background” (i.e., not wait for them to finish), you can use the popen method, which returns a normal subprocess.Popen object:

>>> p = ls.popen("-a")
>>> p.communicate()
('.\n..\n.git\n.gitignore\n.project\n.pydevproject\nREADME.rst\nplumbum\n[...]', '')

You can read from its stdout, wait() for it, terminate() it, etc.

Background and Foreground

In order to make programming easier, there are two special objects called FG and BG, which are there to help you. FG runs programs in the foreground (they receive the parent’s stdin, stdout and stderr), and BG runs programs in the background (much like popen above, but it returns a Future object, instead of a subprocess.Popen one). FG is especially useful for interactive programs like editors, etc., that require a TTY or input from the user.

>>> from plumbum import FG, BG
>>> ls["-l"] & FG
total 5
-rw-r--r--    1 sebulba  Administ     4478 Apr 29 15:02 README.rst
drwxr-xr-x    2 sebulba  Administ     4096 Apr 27 12:18 plumbum
-rw-r--r--    1 sebulba  Administ        0 Apr 27 11:54 setup.py
drwxr-xr-x    2 sebulba  Administ        0 Apr 27 11:54 tests
-rw-r--r--    1 sebulba  Administ       18 Apr 27 11:54 todo.txt

Note

The output of ls went straight to the screen

>>> ls["-a"] & BG
<Future ['C:\\Program Files\\Git\\bin\\ls.exe', '-a'] (running)>
>>> f = _
>>> f.ready()
False
>>> f.wait()
>>> f.stdout
'.\n..\n.git\n.gitignore\n.project\n.pydevproject\nREADME.rst\nplumbum\n[...]'

If you want to redirect the output, you can pass those arguments to the BG modifier. So the command ls & BG(stdout=sys.stdout, stderr=sys.stderr) has exactly the same effect as ls & in a terminal.

You can also start a long running process and detach it in nohup mode using the NOHUP modifier:

>>> ls["-a"] & NOHUP

If you want to redirect the input or output to something other than nohup.out, you can add parameters to the modifier:

>>> ls["-a"] & NOHUP(stdout='/dev/null') # Or None

New in version 1.6: The NOHUP modifier

You can also use the TEE modifier, which causes output to be redirected to the screen (like FG), but also provides access to the output (like BG).

Command Nesting

The arguments of commands can be strings (or any object that can meaningfully-convert to a string), as we’ve seen above, but they can also be other commands! This allows nesting commands into one another, forming complex command objects. The classic example is sudo:

>>> from plumbum.cmd import sudo
>>> print sudo[ls["-l", "-a"]]
/usr/bin/sudo /bin/ls -l -a

>>> sudo[ls["-l", "-a"]]()
u'total 22\ndrwxr-xr-x    8 sebulba  Administ     4096 May  9 20:46 .\n[...]'

In fact, you can nest even command-chains (i.e., pipes and redirections), e.g., sudo[ls | grep["\\.py"]]; however, that would require that the top-level program be able to handle these shell operators, and this is not the case for sudo. sudo expects its argument to be an executable program, and it would complain about | not being one. So, there’s a inherent differnce between between sudo[ls | grep["\\.py"]] and sudo[ls] | grep["\\.py"] (where the pipe is unnested) – the first would fail, the latter would work as expected.

Some programs (mostly shells) will be able to handle pipes and redirections – an example of such a program is ssh. For instance, you could run ssh["somehost", ls | grep["\\.py"]](); here, both ls and grep would run on somehost, and only the filtered output would be sent (over SSH) to our machine. On the other hand, an invocation such as (ssh["somehost", ls] | grep["\\.py"])() would run ls on somehost, send its entire output to our machine, and grep would filter it locally.

We’ll learn more about remote command execution later. In the meanwhile, we should learn that command nesting works by shell-quoting (or shell-escaping) the nested command. Quoting normally takes place from the second level of nesting:

>>> print ssh["somehost", ssh["anotherhost", ls | grep["\\.py"]]]
/bin/ssh somehost /bin/ssh anotherhost /bin/ls '|' /bin/grep "'\\.py'"

In this example, we first ssh to somehost, from it we ssh to anotherhost, and on that host we run the command chain. As you can see, | and the backslashes have been quoted, to prevent them from executing on the first-level shell; this way, they would safey get to the second-level shell.

For further information, see the api docs.