shellrunner 0.4.2

Write safe shell scripts in Python.

ShellRunner

Write safe shell scripts in Python.
Combine the streamlined utility of a shell with the power of a modern programming language.

---

• Install
• Usage
• Why?
  • Similar Projects
• Advanced Usage
  • Shell Command Result
  • Exception Handling
  • Multiple Commands / Persisting Environment
• Options
  • Output
  • Environment Variables
• Examples

Install

pip install -U shellrunner

Usage

from shellrunner import X

X("echo hello world")
# hello world

Easily get a command's output, do something with it, and run another command using the value:

output = X("echo hello world | sed 's/world/there/'").out
greeting = output.capitalize()
X(f"echo 'echo {greeting}' >> .bashrc")

An exception is raised if a command exits with a non-zero status (like bash's set -e):

text = X("grep hello /non/existent/file").out # grep exits with a non-zero status
# ^ Raises ShellCommandError so the rest of the script doesn't run
my_text_processor(text)

Or, maybe you want to handle the error:

from shellrunner import X, ShellCommandError

text = ""
try:
    text = X("grep hello /non/existent/file").out
except ShellCommandError:
    text = X("grep hello /file/that/definitely/exists").out
my_text_processor(text)

Pipeline errors are not masked (like bash's set -o pipefail):

X("grep hello /non/existent/file | tee new_file") # tee gets nothing from grep, creates an empty file, and exits with status 0
# ^ Raises ShellCommandError

Why?

Why not just use bash with set -e and set -o pipefail?

Because writing anything remotely complicated in bash kinda sucks :)

One of the primary advantages of ShellRunner's approach is that you can seamlessly swap between the shell and Python. Some things are just easier to do in a shell (e.g. pipelines) and a lot of things are easier/better in Python (control flow, error handling, etc).

Also, users of fish might know that it does not offer a way to easily exit a script if a command fails. ShellRunner adds set -e and pipefail like functionality to any shell. Leverage the improved syntax of your preferred shell and the (optional) safety of bash.

Similar Projects

• zxpy
• shellpy
• plumbum

ShellRunner is very similar to zxpy and shellpy but aims to be more simple in its implementation and has a focus on adding safety to scripts.

Advanced Usage

A note on compatibility: ShellRunner should work with on any POSIX-compliant system (and shell). No Windows support at this time.

Confirmed compatible with sh (dash), bash, zsh, and fish.

Commands are automatically run with the shell that invoked your python script (this can be overridden):

# my_script.py
X("echo hello | string match hello")
# Works if my_script.py is executed under fish (string match). Will obviously fail if using bash.

Shell Command Result

X returns a ShellCommandResult (NamedTuple) containing the following:

• out: str: The stdout and stderr of the command.
• status: int: The overall exit status of the command. If the command was a pipeline that failed, status will be equal to the status of the last failing command (like bash's pipefail).
• pipestatus: list[int]: A list of statuses for each command in the pipeline.

result = X("echo hello")
print(f'Got output "{result.out}" with exit status {result.status} / {result.pipestatus}')
# Or unpack
output, status, pipestatus = X("echo hello")
# output = "hello"
# status = 0
# pipestatus = [0]

result = X("(exit 1) | (exit 2) | echo hello")
# result.out = "hello"
# result.status = 2
# result.pipestatus = [1, 2, 0]

If using a shell that does not support PIPESTATUS such as sh, you will only ever get the status of the last command in a pipeline. This also means that in this case ShellRunner cannot detect if an error occurred in a pipeline:

result = X("(exit 1) | echo hello")
# if invoked with bash: ShellCommandError is raised, status = 1, pipestatus = [1, 0]
# if invoked with sh: No exception is raised, status = 0, pipestatus = [0]

Exception Handling

ShellCommandError also receives the information from the failed command, which means you can do something like this:

try:
    X("echo hello && false") # Pretend this is some command that prints something but exits with a non-zero status
except ShellCommandError as e:
    print(f'Command failed. Got output "{e.out}" with exit status {e.status}')

Multiple Commands / Persisting Environment

Each call of X invokes a new instance of the shell, so things like environment variables or directory changes don't persist.

Sometimes you might want to do something like this:

X("MY_VAR=hello")
X("grep $MY_VAR /file/that/exists") # MY_VAR doesn't exist
# ^ Raises ShellCommandError

A (bad) solution would be to do this:

X("MY_VAR=hello; grep $MY_VAR /file/that/exists")

This sort of defeats the purpose of ShellRunner because that would be run as one command, so no error handling can take place on commands before the last one.

Instead, X also accepts a list of commands where each command is run in the same shell instance and goes through the normal error handling:

X([
"MY_VAR=hello",
"grep $MY_VAR /file/that/exists",
])
# Works!

Options

There are a few keyword arguments you can provide to adjust the behavior of X:

X("command", shell="bash", check=True, show_output=True, show_command=True)

shell: str (Default: the invoking shell) - Shell that will be used to execute the commands. Can be a path or simply the name (e.g. "/bin/bash", "bash").

check: bool (Default: True) - If True, an error will be thrown if a command exits with a non-zero status.

show_output: bool (Default: True) - If True, command output will be printed.

show_command: bool (Default: True) - If True, the current command will be printed before execution.

Output

Say you do this:

X("echo hello world")

This will print the following to your terminal:

shellrunner: echo hello world
hello world

To hide the shellrunner: lines, set show_command=False.

To hide actual command output, set show_output=False.

Environment Variables

Each option also has a corresponding environment variable to allow you to set these options "globally" for your script:

shell = SHELLRUNNER_SHELL

check = SHELLRUNNER_CHECK

show_output = SHELLRUNNER_SHOW_OUTPUT

show_command = SHELLRUNNER_SHOW_COMMAND

Environment variables are evaluated on each call of X, so you could also do something like this:

# Pretend that before running this file you set: export SHELLRUNNER_SHOW_OUTPUT="False"
X("echo hello")
# No output

# Now you want to see output
os.environ["SHELLRUNNER_SHOW_OUTPUT"] = "True"
X("echo hello")
# hello

Examples

Prints out installed python packages and their dependencies:

from shellrunner import X

packages = X("pip list -l | sed 1,2d | awk '{print $1}'").out
packages = packages.splitlines()

for package in packages:
    print(f"=== {package} ===")
    X(f"pip show {package} | grep -E 'Requires|Required-by'", show_command=False)