FAQ

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.

See also

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:

  1. To make programs behave in the same way as seen on the commandline.
  2. 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.

See also

Buffers

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 pager (typically 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 SIGPIPE is received.

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 glob.glob() first:

import sh
import glob
sh.ls(glob.glob("*.py"))

How do I call a program that isn’t in $PATH?

Use the 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 google-chrome http://google.com

Note

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 . have 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 sh.cat incrementally.

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 block, _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

See also

_fg

You can use sys.stdin, sys.stdout, and sys.stderr as arguments to _in, _out, _err, respectively, and it should mostly work as expected:

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 by newlines.

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 them.

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, "/home/oh", "no", "a", and "space" would 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 arg2 cannot 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 specific order.

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 sh.command invocations, 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 pylintrc:

[TYPECHECK]
generated-members=sh

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:

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

Pros:

  • 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.