Improve background job.nu (#607)

* add Q&A session in README, and add `--label`, `--group` flag to spawn, add `output` sub command to acquire output easily

* make detailed to be a flag

modules/background_task/README.md

@@ -22,3 +22,64 @@ use job.nu
 # clean pueued finished job
 > job clean
 ```
+
+# Q&A
+## How can I pass data to background job?
+
+The easiest way to do is pass it by environment variable, and you can use these variables in job.
+
+So don't do this, it doesn't work:
+```nushell
+let a = 3
+job spawn {
+    echo $a
+}
+```
+
+Instead, doing this:
+```nushell
+let a = 3
+with-env {"a": 3} {
+    job spawn {
+        echo $env.a
+    }
+}
+```
+
+But what if I want to pass nushell's structure data to background job?
+
+You can serialize it to env variable, and then deserialize it in background job.
+
+```nushell
+let a = [1, 2, 3]
+with-env {"a": ($a | to json)} {
+    job spawn {
+        let a = ($env.a | from json)
+        echo $a
+    }
+}
+```
+
+## How can I reuse custom commands in background job?
+
+You can defined these custom commands in a module, then you can use the module inside the job.
+
+So don't do this, it won't work:
+```nushell
+def custom [] { echo 3 }
+job spawn {
+    custom      # custom is not defined
+}
+```
+
+Instead, do this:
+```nushell
+# a.nu
+export def custom [] { echo 3 }
+
+# main.nu
+job spawn {
+    use a.nu
+    a custom
+}
+```

modules/background_task/job.nu

@@ -3,15 +3,29 @@
 # please note that a fresh nushell is spawned to execute the given command
 # So it doesn't inherit current scope's variables, custom commands, alias definition, except env variables which value can convert to string.
 #
+# Note that the closure to spawn can't take arguments.  And it only supports something like this: { echo 3 }, it have no parameter list.
+#
 # e.g:
 # spawn { echo 3 }
 export def spawn [
     command: closure   # the command to spawn
+    --label(-l): string    # the label of comand
+    --group(-g): string    # the group name to spawn
 ] {
     let config_path = $nu.config-path
     let env_path = $nu.env-path
     let source_code = (view source $command | str trim -l -c '{' | str trim -r -c '}')
-    let job_id = (pueue add -p $"nu --config \"($config_path)\" --env-config \"($env_path)\" -c '($source_code)'")
+    mut args = [
+        $"nu --config \"($config_path)\" --env-config \"($env_path)\" -c '($source_code)'",
+    ]
+    if $label != null {
+        $args = ($args | prepend ["--label", $label])
+    }
+    if $group != null {
+        $args = ($args | prepend ["--group", $group])
+    }
+    let job_id = (pueue add -p $args)
+
     {"job_id": $job_id}
 }
 
@@ -26,14 +40,31 @@ export def log [
     | flatten status
 }
 
+# get job's stdout.
+export def output [
+    id: int   # id to fetch job's stdout
+] {
+    log $id | get output.0
+}
+
 # get job running status
-export def status () {
+export def status [
+    --detailed(-d)   # need to get detailed stauts?
+] {
+    let output = (
         pueue status --json
         | from json
         | get tasks
         | transpose -i status
         | flatten
         | flatten status
+    )
+
+    if not $detailed {
+        $output | select id label group Done? status? start? end?
+    } else {
+        $output
+    }
 }
 
 # kill specific job