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Script Input, File Writes, and OS Controls

· 3 min read
Erik Osterman
Erik Osterman
Founder @ Cloud Posse

Sending a secret to a subprocess should not require putting it in command-line arguments. Generating a local configuration file or reading an inherited environment variable should not require launching another program. Atmos automation scripts now support standard input, file writes, and environment lookups directly, with script-local environment and directory controls.

The Problem​

Scripts could start programs and read files, but could not supply a program's standard input or write a file. The script's explicit env inputs also differed from the inherited environment available to its subprocesses. That left scripts passing values as command arguments or relying on helper programs for ordinary input and output.

The Fix​

The process functions and Atmos command wrappers accept stdin, file writes use fs.write_file, and the predeclared os module exposes the effective environment, current directory, platform, and architecture. Scripts can change their own execution defaults without changing the host Atmos process. The env dictionary still contains only explicit step inputs, and process calls without stdin receive EOF.

How to Use It​

Read a token and send it through standard input:

token = os.getenv("TOKEN")
if token == None:
    fail("TOKEN is required")
exec.run(["some-tool", "--token-stdin"], stdin = token, output = "capture")

Input is sent exactly as supplied, without an added newline. Each retry receives the complete input again. Atmos excludes the payload from command diagnostics; the subprocess still controls its own output. The same stdin option is available on component process calls and all Atmos command wrappers, including atmos.secret. Pass any CLI flag that tells the receiving command to read stdin separately; the wrapper does not add that flag automatically.

Generate a local configuration file:

fs.write_file("config.json", json.encode({"region": os.getenv("AWS_REGION", "us-east-1")}))

New files default to owner-only read/write permissions. The parent directory must exist, and an existing file causes an error unless you pass overwrite = True. Overwriting preserves the file's existing permissions.

Inspect and adjust execution defaults:

original_directory = os.getcwd()
original_environment = os.environ()
print(os.platform, os.arch)
os.setenv("AWS_REGION", "us-east-1")
os.chdir("generated")  # This directory must already exist.
fs.write_file("config.json", json.encode({"region": os.getenv("AWS_REGION")}))
os.chdir(original_directory)

The os.environ() function returns a frozen snapshot; later changes do not alter it. The os.platform and os.arch attributes describe the running executable. The os.getcwd() function reads the script's current directory.

Call os.setenv() and os.chdir() only in the main thread. Parallel tasks inherit those settings and use per-call env= or working_directory= for their own overrides. Environment changes apply to subsequent process and step calls, including existing component handles. Directory changes affect relative file operations and default process and step directories; component calls retain their component directory. Neither operation changes the host process or another script invocation, and os.setenv leaves the declared env dictionary unchanged.

Get Involved​

See the automation function reference for the complete APIs. Share your scripting use cases in GitHub Discussions.