ShLib — Shell Library

Ken Kundert





A light-weight package with few dependencies that allows users to do shell-script like things relatively easily in Python. Is a natural complement to the pathlib library. Pathlib does pretty much what you would like to do with a single path; shlib does similar things with many paths at once. For example, with pathlib you can remove (unlink) a single file, but with shlib you can remove many files at once. Furthermore, most of the features of pathlib are implemented as pathlib methods, so you must convert your strings to paths before you can use them. ShLib is equally comfortable with strings as with paths.

Writing programs that substantially interact with the file system can be surprisingly painful in Python because the code that is used to do so is spread over many packages and those packages are not very compatible with each other nor do they follow the conventions of the corresponding shell commands.

This package, shlib, attempts to address those issues by providing one package that combines the commonly used utilities for interacting with the filesystem that follows the conventions used by the corresponding shell commands.

It consists of replacements for some very common Unix utilities that interact with the filesystem, such as cp, mv, rm, ln, mkdir, and cd. These tend to be less fussy than their command line counter parts. For example, rm deletes both files and directories without distinction and will not complain if the file or directory does not exist. Similarly mkdir will create any child directories needed and will not complain if the directory already exists.

Finally, it provides several ways to run external programs.

Each feature is designed to allow you to express your desires simply and efficiently without worrying too much about exceptions.

Most of the functions in this package take paths to files or directories. Those paths may be specified either as strings or pathlib paths. Many of the functions accept multiple paths, and those can be specified either as an array or as individual arguments. Several of the functions return either a path or a collection of paths. These paths are returned as pathlib paths.


Use ‘pip3 install shlib’ to install. Requires Python3.6 or better.

System Utility Functions

Copy (cp)

Copy files or directories:

cp(src, ..., dest)


cp([src, ...], dest)

Copy all source items, whether they be files or directories to dest. If there is more than one src item, then dest must be a directory and the copies will be placed in that directory. The src arguments may be strings, pathlib paths, or collections of strings and paths. The dest must be a string or path.


>>> from shlib import *
>>> testdir = 'testdir'
>>> rm(testdir)
>>> mkdir(testdir)
>>> files = cartesian_product(testdir, ['f1', 'f2'])
>>> touch(files)
>>> dirs = cartesian_product(testdir, ['d1', 'd2'])
>>> mkdir(dirs)
>>> print(sorted(str(e) for e in ls(testdir)))
['testdir/d1', 'testdir/d2', 'testdir/f1', 'testdir/f2']

>>> cp('testdir/f1', 'testdir/f4')
>>> print(sorted(str(f) for f in lsf(testdir)))
['testdir/f1', 'testdir/f2', 'testdir/f4']

>>> dest1 = to_path(testdir, 'dest1')
>>> mkdir(dest1)
>>> cp(files, dest1)
>>> print(sorted(str(f) for f in lsf(dest1)))
['testdir/dest1/f1', 'testdir/dest1/f2']

>>> cp(dirs, dest1)
>>> print(sorted(str(d) for d in lsd(dest1)))
['testdir/dest1/d1', 'testdir/dest1/d2']

>>> f1, f2 = tuple(files)
>>> dest2 = to_path(testdir, 'dest2')
>>> mkdir(dest2)
>>> cp(f1, f2, dest2)
>>> print(sorted(str(f) for f in lsf(dest2)))
['testdir/dest2/f1', 'testdir/dest2/f2']

>>> dest3 = to_path(testdir, 'dest3')
>>> mkdir(dest3)
>>> cp([f1, f2], dest3)
>>> print(sorted(str(f) for f in lsf(dest3)))
['testdir/dest3/f1', 'testdir/dest3/f2']

Move (mv)

Move files or directories:

mv(src, ..., dest)

Move all source items, whether they be files or directories to dest. If there is more than one src item, then dest must be a directory and everything will be placed in that directory. The src arguments may be strings or lists of strings. The dest must be a string.

>>> from shlib import *
>>> testdir = 'testdir'
>>> rm(testdir)
>>> mkdir(testdir)
>>> files = cartesian_product(testdir, ['f1', 'f2'])
>>> touch(files)
>>> dirs = cartesian_product(testdir, ['d1', 'd2'])
>>> mkdir(dirs)
>>> print(sorted(str(e) for e in ls(testdir)))
['testdir/d1', 'testdir/d2', 'testdir/f1', 'testdir/f2']

>>> dest = to_path(testdir, 'dest')
>>> mkdir(dest)
>>> mv(files, dest)                  # move a list of files
>>> print(sorted(str(f) for f in lsf(dest)))
['testdir/dest/f1', 'testdir/dest/f2']

>>> mv(dirs, dest)                   # move a list of directories
>>> print(sorted(str(d) for d in lsd(dest)))
['testdir/dest/d1', 'testdir/dest/d2']

Remove (rm)

Remove files or directories:

rm(path, ...)

Delete all files and directories given as arguments. Does not complain if any of the items do not exist. Each argument must be either a string or a list of strings.

>>> print(sorted(str(e) for e in ls(testdir)))

>>> print(sorted(str(e) for e in ls(dest)))
['testdir/dest/d1', 'testdir/dest/d2', 'testdir/dest/f1', 'testdir/dest/f2']

>>> rm(lsf(dest))
>>> print(sorted(str(e) for e in ls(dest)))
['testdir/dest/d1', 'testdir/dest/d2']

>>> rm(dest)
>>> print(sorted(str(e) for e in ls(testdir)))

>>> rm(testdir)

Make File (touch)

Create a new empty file or update the timestamp on an existing file:

touch(path, ...)

Each argument must be either a string or a list of strings.

Make Directory (mkdir)

Create an empty directory:

mkdir(path, ...)

For each argument it creates a directory and any needed parent directories. Returns without complaint if the directory already exists. Each argument must be either a string or a list of strings.

Change Directory (cd)

Change to an existing directory:


Makes path the current working directory.

May also be used in a with block:

with cd(path):

The working directory returns to its original value upon leaving the with block.

Current Working Directory (cwd)

Returns the current working directory:

path = cwd()

Mount and Unmount a Filesystem (mount)

Mount a filesystem with:


Then unmount it with:


You can test to determine if a filesystem is mounted with:


May also be used in a with block:

with mount(path):
    cp(path/data, '.')

The filesystem is unmounted upon leaving the with block.

List Directory (ls, lsd, lsf)

List a directory:

ls(path, ... [<kwargs>])
lsd(path, ... [<kwargs>])
lsf(path, ... [<kwargs>])

The first form returns a list of all items found in a directory. The second returns only the directories, and the third returns only the files.

One or more paths may be specified using unnamed arguments. The paths may be strings or pathlib paths, or collections of those. If no paths are not given, the current working directory is assumed.

The remaining arguments must be specified as keyword arguments.


If select is specified, an item is returned only if it matches the given pattern. Using ‘**’ in select enables a recursive walk through a directory and all its subdirectories. Using ‘**’ alone returns only directories whereas ‘**/*’ returns files and directories.


If reject is specified, an item is not returned if it matches the given pattern.


If only is specified, it may be either ‘file’ or ‘dir’, in which case only items of the corresponding type are returned.


The value of hidden is a boolean that indicates whether items that begin with ‘.’ are included in the output. If hidden is not specified, hidden items are not included unless select begins with ‘.’.


pyfiles = lsf(select='*.py')
subdirs = lsd()
tmp_mutt = lsf('/tmp/', select='mutt-*')

File Permissions

Change the file permissiongs of a file, or files, or directory, or directories:

chmod(mode, path)

where mode is a three digit octal number.

You may read the permissions of a file or directory using:

mode = getmod(path)



Create a path from a collection of path segments:

p = to_path(seg, ...)

The segments are combined to form a path. Expands a leading ~. Returns a pathlib path. It is generally not necessary to apply to_path() to paths being given to the shlib functions, but using it gives you access to all of the various pathlib methods for the path.

>>> path = to_path('A', 'b', '3')
>>> str(path)

to_path returns a Path object that has been extended from the standard Python pathlib Path object. Specifically, it includes the following methods:

p.is_readable()   — return True if path exists and is readable
p.is_writable()   — return True if path exists and is writable
p.is_executable() — return True if path exists and is executable
p.is_hidden()     — return True if path exists and is hidden (name starts with .)
p.is_newer()      — return True if path exists and is newer than argument
p.path_from()     — differs from relative_to() in that returned path will not start with ..
p.sans_ext()      — return full path without the extension

See extended_pathlib for more information.


Recursively descend into a directory yielding paths to all of the files it contains. Normally hidden files are excluded unless the hidden argument is True. OSErrors found during the scan are ignored unless the report argument is specified, and if specified it must be a function that takes one argument, the exception raised by the error.

Cartesian Product

Create a list of paths by combining from path segments in all combinations:

cartesian_product(seg, ...)

Like with to_path(), the components are combined to form a path, but in this case each component may be a list. The results is the various components are combined in a Cartesian product to form a list. For example:

>>> paths = cartesian_product(['A', 'B'], ['a', 'b'], ['1', '2'])
>>> for p in paths:
...     print(p)

Brace Expand

Create a list of paths using Bash-like brace expansion:

>>> paths = brace_expand('python{2.{5..7},3.{2..6}}')

>>> for p in sorted(str(p) for p in paths):
...     print(p)

Executing Programs

The following classes and functions are used to execute external programs from within Python.

Command (Cmd)

A class that runs an external program:

Cmd(cmd[, modes][, env][, encoding][, log][, option_args])

cmd may be a list or a string. mode is a string that specifies various options. The options are specified using a single letter, with upper case enabling the option and lower case disabling it:

S, s: Use, or do not use, a shell
O, o: Capture, or do not capture, stdout
E, e: Capture, or do not capture, stderr
M, m: Merge, or do not merge, stderr into stdout (M overrides E, e)
W, w: Wait, or do not wait, for command to terminate before proceeding

If a letter corresponding to a particular option is not specified, the default is used for that option. In addition, one of the following may be given, and it must be given last

*: accept any output status code
N: accept any output status code equal to or less than N
M,N,…: accept status codes M, N, …

If you do not specify the status code behavior, only 0 is accepted as normal termination, all other codes will be treated as errors. An exception is raised if exit status is not acceptable. By default an OSError is raised, however if the use_inform preference is true, then inform.Error is used. In this case the error includes attributes that can be used to access the stdout, stderr, status, cmd, and msg.

env is a dictionary of environment variable and their values.

encoding is used on the input and output streams when converting them to and from strings.

log specifies whether details about the command should be sent to log file. May be True, False, or None. If None, then behavior is set by log_cmd preference. Use of log requires that Inform package be installed.

option_args is used when rendering command to logfile, it indicates how many arguments each option takes. This only occurs when use_inform preference is true and Inform package is installed.

For example, to run diff you might use:

>>> import sys, textwrap
>>> ref = textwrap.dedent('''
...     line1
...     line2
...     line3
... ''').strip()
>>> test = textwrap.dedent('''
...     line1
...     line2
... ''').strip()

>>> ref_bytes_written = to_path('./REF').write_text(ref)
>>> test_bytes_written = to_path('./TEST').write_text(test)

>>> cat = Cmd(['cat', 'TEST'], 'sOeW')

>>> print(cat.stdout)

>>> diff = Cmd('diff TEST REF', 'sOEW1')
>>> status =
>>> status

Use of O in the modes allows access to stdout, which is needed to access the differences. Specifying E also allows access to stderr, which in this case is helpful in case something goes wrong because it allows the error handler to access the error message generated by diff. Specifying W indicates that run() should block until diff completes. This is also necessary for you to be able to capture either stdout or stderr. Specifying 1 indicates that either 0 or 1 are valid output status codes; any other code output by diff would be treated as an error.

If you do not indicate that stdout or stderr should be captured, those streams remain connected to your TTY. You can specify a string to the run() method, which is fed to the program through stdin. If you don’t specify anything the stdin stream for the program also remains connected to the TTY.

If you indicate that run() should return immediately without out waiting for the program to exit, then you can use the wait() and kill() methods to manage the execution. For example:

diff = Cmd(['gvim', '-d', lfile, rfile], 'w')
    status = diff.wait()
except KeyboardInterrupt:

Casting the object to a string returns the command itself:

>>> print(str(cat))
cat TEST

If you call run(), then you should either specify ‘W’ as the wait mode, or you should call the wait() method. If you do not, then any string you specified as stdin is not applied. If your intention is to kick off a process and not wait for it to finish, you should use start() instead. It also allows you to specify a string to pass to stdin, however you cannot access stdout, stderr, or the exit status. If you specify the ‘O’ or ‘E’ modes when using start(), those outputs are simply discarded. This is a useful way of discarding uninteresting diagnostics from the program you are calling.

Cmd also provides the render method, which converts the command to a string. It takes the same optional arguments as does render_command.


Run subclasses Cmd. It basically constructs the process and then immediately calls the run() method. It takes the same arguments as Cmd, but an additional argument that allows you to specify stdin for the process:

Run(cmd[, modes][, stdin][, env][, encoding])

Run expect you to wait for the process to end, either by specify the ‘W’ mode, or by calling wait(). For example:

>>> echo = Run('cat > helloworld', 'SoeW', 'hello world')
>>> echo.status

>>> echo = Run(['echo', 'helloworld'], 'sOew')
>>> echo.wait()

>>> print(echo.stdout.strip())


Start also subclasses Cmd. It is similar to Run in that it immediately executes the command, but it differs in that it does not expect you to wait for the command to terminate. You may specify stdin to the command if you wish, but since you are not waiting for the command to terminate you cannot access stdout, stderr or the exit status. Effectively, Start() kicks off the process and then ignores it. You may pass wait or accept in the mode string, but they are ignored. If you select either stdout or stderr to be captured, then are wired to /dev/null, meaning that the selected output is swallowed and discarded.

>>> cat = Start('cat helloworld', 'sOe')


Given a name, a path, and a collection of read, write, or execute flags, this function returns the locations along the path where a file or directory can be found with matching flags:

which(name, path=None, flags=os.X_OK)

By default the path is specified by the PATH environment variable and the flags check whether you have execute permission.


Render a command to a string:

render_command(cmd[, option_args][, width])

Converts the command to a string. The formatting is such that you should be able to feed the result directly to a shell and have command execute properly.

cmd is the command to render. It may be a string or a list of strings.

option_args is a dictionary. The keys are options accepted by the command and the value is the number of arguments for that option. If an option is not found, it is assumed to have 0 arguments.

width specifies how long the string must be before it is broken into multiple lines. If length of resulting line would be width or less, return as a single line, otherwise place each argument and option on separate line.

If the command is rendered as multiple lines, each argument and option is placed on a separate line, while keeping argument to options on the same line as the option. Placing each option and argument on its own line allows complicated commands with long arguments to be displayed cleanly.

For example:

>>> args = {'--dux': 2, '-d': 2, '--tux': 1}
>>> print(render_command('bux --dux a b -d c d --tux e f g h', args))
bux --dux a b -d c d --tux e f g h

>>> print(render_command('bux --dux a b -d c d --tux e f g h', args, width=0))
bux \
    --dux a b \
    -d c d \
    --tux e \
    f \
    g \


Used to set preferences that affect the Cmd class. The preferences are given as keyword arguments.

use_inform indicates that the Inform exception Error should be raised if the exit status from a command is not acceptable. If this not given or is False, an OSError is raised instead. Use of this preference requires that Inform be available. If use_inform is True, then inform.Error() is used by Cmd and its subclasses (Run and Start).

log_cmd specifies that the command and its exit status should be written to the Inform log file. Use of this preference requires that Inform be available.

Error Reporting with Inform

The Cmd class and its subclasses (Run and Start) raise an Inform Error if the use_inform preference was specified. This allows for rich error reporting. In particular, the command, exit status, stdout and stderr are all returned with the exception and are available to insert into an error message. For example:

>> from shlib import Run, set_prefs
>> from inform import Error

>> set_prefs(use_inform=True)

>> try:
..     c = Run('sort words', 'sOEW0')
.. except Error as e:
..         '"{cmd}" exits with status {status}.\n    {stderr}',
..         '"{cmd}" exits with status {status}.',
..     ))
error: "sort words" exits with status 2.
    sort: cannot read: words: No such file or directory.

If command returns a non-zero exit status, an exception is raised and one of two error messages are printed. The first is printed if stderr is not empty, and the second is printed if it is.

Most other functions raise an OSError upon an error. You can use Inform to convert this exception into a reasonable error message:

>> from inform import fatal, os_error
>> try:
..    cp(from, to)
.. except OSError as e:
..    fatal(os_error(e))