jj-run: add JJ_CHANGE_ID, JJ_COMMIT_ID, JJ_COMMIT_TIMESTAMP env vars

When running commands, jj-run now sets environment variables with
metadata about the current change being processed:

- JJ_CHANGE_ID: The change ID (jj's unique identifier)
- JJ_COMMIT_ID: The full commit hash
- JJ_COMMIT_TIMESTAMP: ISO 8601 timestamp of the commit

These variables are available in all modes (workspace, direct, readonly)
and are useful for logging, sorting results, and correlating output
when running commands in parallel.

Author: neongreen

jj-run/README.md

@@ -117,6 +117,30 @@ jj x --readonly -r '::@' 'cargo build'
 jj x --readonly -r 'recent(5)' 'cargo llvm-cov --html'
 ```
 
+## Environment Variables
+
+When running commands, jj-run sets the following environment variables with metadata about the current change being processed:
+
+| Variable | Description | Example |
+|----------|-------------|---------|
+| `JJ_CHANGE_ID` | The change ID (jj's unique identifier) | `plupmpozpuunnxwmxvsussklkvqlsmul` |
+| `JJ_COMMIT_ID` | The full commit hash | `162b3b24f7088209c33630ec56deefebc70c2128` |
+| `JJ_COMMIT_TIMESTAMP` | ISO 8601 timestamp of the commit | `2026-01-18T02:55:02+01:00` |
+
+These variables are available in all modes (workspace, direct, and readonly).
+
+**Example usage:**
+```sh
+# Print change info for each commit
+jj x --readonly -r 'mutable()' 'echo "$JJ_COMMIT_TIMESTAMP $JJ_CHANGE_ID"'
+
+# Run tests and log results with timestamps (useful when results arrive out of order)
+jj x --readonly -j4 -r 'ancestors(@, 20)' '
+  echo "=== $JJ_COMMIT_TIMESTAMP $JJ_CHANGE_ID ==="
+  cargo test 2>&1 | grep -E "(Passed|Failed|Total)"
+'
+```
+
 ## Working with Remote Repositories
 
 **⚠️ EXPERIMENTAL: This feature is AI-written and untested. Use at your own risk.**

jj-run/main.go

@@ -16,12 +16,20 @@ import (
 	"github.com/spf13/cobra"
 )
 
+// Signature represents author/committer information
+type Signature struct {
+	Name      string `json:"name"`
+	Email     string `json:"email"`
+	Timestamp string `json:"timestamp"`
+}
+
 // Change represents a jj change with its metadata
 type Change struct {
-	CommitID    string   `json:"commit_id"`
-	ChangeID    string   `json:"change_id"`
-	Description string   `json:"description"`
-	Parents     []string `json:"parents"`
+	CommitID    string    `json:"commit_id"`
+	ChangeID    string    `json:"change_id"`
+	Description string    `json:"description"`
+	Parents     []string  `json:"parents"`
+	Committer   Signature `json:"committer"`
 }
 
 // Job represents a change processing job
@@ -303,7 +311,7 @@ func runDirectMode(command string, strategy ErrorStrategy, beforeOp string, repo
 		}
 
 		// Run the command in the repository
-		err := runShellCommand(command, repoDir)
+		err := runShellCommand(command, repoDir, change)
 		printCommandResult(err)
 
 		if err != nil {
@@ -394,7 +402,7 @@ func runReadonlyMode(command string, strategy ErrorStrategy, repoDir string) err
 		}
 
 		// Run the command in the workspace
-		err = runShellCommand(command, workspacePath)
+		err = runShellCommand(command, workspacePath, change)
 		printCommandResult(err)
 
 		// After command execution, use --ignore-working-copy for all jj commands
@@ -607,7 +615,7 @@ func processChanges(workspacePath string, changes []*Change, command string, str
 		}
 
 		// Run the command
-		err := runShellCommand(command, workspacePath)
+		err := runShellCommand(command, workspacePath, change)
 		printCommandResult(err)
 
 		if err != nil {
@@ -693,7 +701,7 @@ func processChangesParallel(changes []*Change, command string, strategy ErrorStr
 				}
 
 				// Run the command
-				err := runShellCommand(command, workspacePath)
+				err := runShellCommand(command, workspacePath, change)
 				printCommandResult(err)
 
 				if err != nil {
@@ -855,12 +863,23 @@ func printCommandResult(err error) {
 	fmt.Println()
 }
 
-func runShellCommand(command string, cwd string) error {
+// runShellCommand runs a shell command in the given directory.
+// If change is provided, it sets JJ_CHANGE_ID, JJ_COMMIT_ID, and JJ_COMMIT_TIMESTAMP env vars.
+func runShellCommand(command string, cwd string, change *Change) error {
 	cmd := exec.Command("sh", "-c", command)
 	cmd.Dir = cwd
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr
 
+	// Set environment variables if change metadata is provided
+	if change != nil {
+		cmd.Env = append(os.Environ(),
+			"JJ_CHANGE_ID="+change.ChangeID,
+			"JJ_COMMIT_ID="+change.CommitID,
+			"JJ_COMMIT_TIMESTAMP="+change.Committer.Timestamp,
+		)
+	}
+
 	return cmd.Run()
 }