krastanov/julia-external-cmd

Julia External Cmd

Use Julia backtick command literals and Cmd APIs to run subprocesses without shell ambiguity.

Core Command Model

Backticks produce Cmd, they do not execute immediately:

cmd = `echo hello`
run(cmd)
text = readchomp(cmd)

Remember: Julia executes commands directly (no implicit shell), so quoting/interpolation follows Julia command rules, not shell expansion.

Interpolation and Quoting

• Use $value to pass one argument safely, including paths with spaces.
• Interpolate arrays/iterables to expand into multiple arguments.
• Inside one shell word, array interpolation performs shell-like brace/cartesian expansion.
• Quote shell metacharacters inside backticks when meant literally.

Build Pipelines Explicitly

Use pipeline instead of shell metacharacters in command literals:

run(pipeline(`cut -d: -f3 /etc/passwd`, `sort -n`, `tail -n5`))

Use & to run producers in parallel and merge output into downstream stages.

Avoid Pipeline Deadlocks

When both writing and reading, actively consume output:

• Prefer read(out, String) over waiting on process completion alone.
• Split reader/writer into separate tasks when needed.

Configure Command Environment

Use Cmd(...) keywords and env helpers:

• Cmd(cmd; dir=..., env=..., detach=..., ignorestatus=...)
• setenv(cmd, pairs...)
• addenv(cmd, pairs...)

Use open(cmd, mode, ...) for stream-style interaction.

Platform Note

Many shell built-ins on Windows are not external executables; invoke cmd /C ... when necessary.

Reference

• references/cmd-pipeline-patterns.md - command composition and subprocess IO patterns

Related Skills

• julia-async - task-based orchestration around process IO
• julia-comonicon - Julia CLI development once command patterns are stable