Job Control

For what job control is, see Effective Shell/Job-Control#What-is-job-control.

In short, it is the ability to move tasks between the foreground and the background.

Problems It Solves

1. Freeing the terminal: move a long-running task into the background so the command line doesn’t get stuck, letting you multitask in a single window.
2. Suspend and resume: pause the current program (e.g. Vim) to handle something urgent, then seamlessly return to where you were.
   P.S: if you use Tmux/Zellij, a floating pane also works for handling interruptions, see my post on that
3. Task management: list every task, know exactly what is running, and kill any of them at will.

Job Management Overview

Nushell’s built-in job command provides functionality similar to UNIX shells. Run help job to see how it works.

You could say Nushell and PowerShell commands are quite self-explanatory. UNIX shells, by contrast, lean toward brevity, relying heavily on symbols and abbreviations that you have to memorize, which makes them hard for beginners to decipher.

$ help jobVarious commands for working with background jobs.You must use one of the following subcommands. Using this command as-is will only produce this help message.Usage:  > jobSubcommands:  job flush - Clear this job's mailbox.  job id - Get id of current job.  job kill - Kill a background job.  job list - List background jobs.  job recv - Read a message from the mailbox.  job send - Send a message to the mailbox of a job.  job spawn - Spawn a background job and retrieve its ID.  job tag - Add a description tag to a background job.  job unfreeze - Unfreeze a frozen process job in foreground.Flags:  -h, --help: Display the help message for this commandInput/output types:  ╭───┬─────────┬────────╮  │ # │  input  │ output │  ├───┼─────────┼────────┤  │ 0 │ nothing │ string │  ╰───┴─────────┴────────╯

End of post, below I’ll pick a few subcommands I find interesting.

Spawn Background Jobs🫃🏻

Spawn [spɔːn]🔊 means to hatch; in Nushell it means starting a background job.

• P.S. I first ran into this word while modding Cyberpunk, where spawning NPCs uses exactly this word (😡why aren’t this NPC’s clothes loading)

Usage needs little explanation, so straight to an example. hexo clean; hexo gen takes ~10s and blocks the prompt while it runs, so it’s more convenient to push it into the background:

# Start a background job$ job spawn { hexo clean; hexo gen }# Confirm it is quietly working in the background$ job list╭───┬────┬────────┬──────────────╮│ # │ id │  type  │     pids     │├───┼────┼────────┼──────────────┤│ 0 │ 34 │ thread │ ╭───┬──────╮ ││   │    │        │ │ 0 │ 5359 │ ││   │    │        │ ╰───┴──────╯ │╰───┴────┴────────┴──────────────╯

Care about the job’s output? I haven’t found a corresponding Nushell API yet; one workaround is to redirect stdout/stderr to files:
hexo generate o> hexo.log e> hexo.err

Process Bound

When you run job spawn { ... }, Nushell actually starts a new Rust thread inside the current shell process to run the closure^[1], which means:

• Jobs in different Nushell processes are independent of each other; you can’t manage them across processes
• When you exit Nushell, background jobs get cleaned up too:

$ exitThere are still background jobs running (1).Running `exit` a second time will kill all of them.

Usage Scenarios

• Uploading files
  • A concrete example: Atuin’s Nushell script uses job spawn to create an async upload task ^[2]
• Downloading files
• Compile Code etc.

Freeze 🥶

Just like in a UNIX shell, you can freeze a running foreground process:

1. Run hx ./test.js in the foreground
2. Press Ctrl+Z or send the process a SIGTSTP signal; Nushell reports: Job 20 is frozen
3. Do something else…
4. Use job unfreeze 20 to bring it back to the foreground and resume

A frozen job no longer consumes CPU, but it still occupies memory and keeps its file handles open, waiting for your next instruction.