Will Windows be supported?¶
There are no plans to support Windows.
How do I append output to a file?¶
Use a file object opened in the mode you desire:
import sh h = open("/tmp/output", "a") sh.ls("/dir1", _out=h) sh.ls("/dir2", _out=h)
Why does my command’s output have color?¶
Typically the reason for this is that your program detected that its STDOUT was connected to a TTY, and therefore decided to print color escape sequences in its output. The typical solution is to use _tty_out=False, which will force a pipe to be connected to STDOUT, and probably change the behavior of the program.
Git is one of the programs that makes extensive use of terminal colors (as well as pagers) in its output, so we added a contrib version for convenience.
Why is _tty_out=True the default?¶
This was a design decision made for two reasons:
- To make programs behave in the same way as seen on the commandline.
- To provide better buffering control than pipes allow.
For #1, we want sh to produce output that is identical to what the user sees from the commandline, because that’s typically the only output they ever see from their command. This makes the output easy to understand.
For #2, using a TTY for STDOUT allows us to precisely control the buffering of a command’s output to sh’s internal code.
Of course, there are some gotchas with TTY STDOUT. One of them is commands that use a pager, for example:
import sh print(sh.git.log())
This will raise a
SignalException_SIGPIPE, and it’s unclear exactly why.
The reason is because
git log detects a TTY STDOUT and forks the system’s
less) to handle the output. The pager checks for a
controlling terminal, and, finding none, exits with exit code 1. The exit of
the pager means no more readers on
git log‘s output, and thus a
The solution to the
git log problem above is simply to use
_tty_out=False, but this is not totally obvious.
Why doesn’t “*” work as a command argument?¶
Glob expansion is a feature of a shell, like Bash, and is performed by the shell before passing the results to the program to be exec’d. Because sh is not a shell, but rather tool to execute programs directly, we do not handle glob expansion like a shell would.
So in order to use
"*" like you would on the commandline, pass it into
import sh import glob sh.ls(glob.glob("*.py"))
How do I call a program that isn’t in
Command() constructor to instantiate an instance of Command
directly, then execute that:
import sh cmd = sh.Command("/path/to/command") cmd("-v", "arg1")
How do I execute a program with a dash in its name?¶
If it’s in your
$PATH, substitute the dash for an underscore:
import sh sh.google_chrome("http://google.com")
The above will run
If a program named
google_chrome exists on your system, that will be
called instead. In that case, in order to execute the program with a dash
in the name, you’ll have to use the method described here.
How do I execute a program with a special character in its name?¶
Programs with non-alphanumeric, non-dash characters in their names cannot be executed directly as an attribute on the sh module. For example, this will not work:
import sh sh.mkfs.ext4()
The reason should be fairly obvious. In Python, characters like
special meaning, in this case, attribute access. What sh is trying to do in the
above example is find the program “mkfs” (which may or may not exist) and then
perform a subcommand lookup with the name “ext4”. In other
words, it will try to call
mkfs with the argument
ext4, which is
probably not what you want.
The workaround is instantiating the Command Class with the string of the program you’re looking for:
import sh mkfsext4 = sh.Command("mkfs.ext4") mkfsext4() # run it
Why not use
| to pipe commands?¶
I prefer the syntax of sh to resemble function composition instead of a pipeline. One of the goals of sh is to make executing processes more like calling functions, not making function calls more like Bash.
Why isn’t piping asynchronous by default?¶
There is a non-obvious reason why async piping is not possible by default. Consider the following example:
import sh sh.cat(sh.echo("test\n1\n2\n3\n"))
When this is run,
sh.echo executes and finishes, then the entire output
string is fed into
sh.cat. What we would really like is each
newline-delimited chunk to flow to
But for this example to flow data asynchronously from echo to cat, the echo command would need to not block. But how can the inner command know the context of its execution, to know to block sometimes but not other times? It can’t know that without something explicit.
This is why the _piped special kwarg was introduced. By default, commands
executed block until they are finished, so in order for an inner command to not
_piped=True signals to the inner command that it should not block.
This way, the inner command starts running, then very shortly after, the outer
command starts running, and both are running simultaneously. Data can then flow
from the inner command to the outer command asynchronously:
import sh sh.cat(sh.echo("test\n1\n2\n3\n", _piped=True))
Again, this example is contrived – a better example would be a long-running command that produces a lot of output that you wish to pipe through another program incrementally.
How do I run a command and connect it to sys.stdout and sys.stdin?¶
There are two ways to do this
import sh import sys sh.your_command(_in=sys.stdin, _out=sys.stdout)
There are a few reasons why this probably won’t work. The first reason is that
sys.stdin is probably a controlling TTY (attached to the shell that
launched the python process), and probably not set in raw mode
termios(3), which means that, among other things, input is buffered
The real solution is to use _fg=True:
import sh sh.top(_fg=True)
Why do my arguments need to be separate strings?¶
This confuses many new sh users. They want to do something like this and expect it to just work:
from sh import tar tar("cvf /tmp/test.tar /my/home/directory")
But instead they’ll get a confusing error message:
RAN: '/bin/tar cvf /tmp/test.tar /my/home/directory' STDOUT: STDERR: /bin/tar: Old option 'f' requires an argument. Try '/bin/tar --help' or '/bin/tar --usage' for more information.
The reason why they expect it to work is because shells, like Bash, automatically
parse your commandline and break up arguments for you, before sending them to
the binary. They have a complex set of rules (some of which are represented by
shlex) to take a single string of a command and arguments and separate
Even if we wanted to implement this in sh (which we don’t), it would hurt the ability for users to parameterize parts of their arguments. They would have to use string interpolation, which would be ugly and error prone:
from sh import tar tar("cvf %s %s" % ("/tmp/tar1.tar", "/home/oh no a space")
In the above example,
all be separate arguments to tar, causing the program to behave unexpectedly.
Basically every command with parameterized arguments would need to expect
characters that could break the parser.
How do I order keyword arguments?¶
Typically this question gets asked when a user is trying to execute something like the following commandline:
my-command --arg1=val1 arg2 --arg3=val3
This is usually the first attempt that they make:
sh.my_command(arg1="val1", "arg2", arg3="val3")
This doesn’t work because, in Python, position arguements, like
come after keyword arguments.
Furthermore, it is entirely possible that
--arg3=val3 comes before
--arg1=val1. The reason for this is that a function’s
**kwargs is an
unordered mapping, and so key-value pairs are not guaranteed to resolve to a
So the solution here is to forego the usage of the keyword argument convenience, and just use raw ordered arguments:
sh.my_command("--arg1=val1", "arg2", "--arg3=val3")
How to disable pylint E1101 no-member errors?¶
Pylint complains with E1101 no-member to almost all
because it doesn’t know, that these members are generated dynamically.
Starting with Pylint 1.6 these messages can be suppressed using generated-members option.
Just add following lines to
Why is sh just a single file?¶
When sh was first written, the design decision was made to make it a single-file module. This has pros and cons:
- Auditing the code is more challenging
- Without file-enforced structure, adding more features and abstractions makes the code harder to follow
- Cognitively, it feels cluttered
- Can be used easily on systems without Python package managers
- Can be embedded/bundled together with other software more easily
- Cognitively, it feels more self-contained
In my mind, because the primary target audience of sh users is generally more scrappy devops, systems people, or people just trying to stitch together some clunky system programs, the listed pros weigh a little more heavily than the cons. Sacrificing some development advantages to give those users a more flexible tool is a win to me.
Down the road, the development disadvantages of a single file can be solved with additional development tools, for example, with a tool that compiles multiple modules into the single sh.py file. Realistically, though, sh is pretty mature, so I don’t see it growing much more in complexity or code size.