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Execute commands in a VM

Just like copying files in and out of a VM, executing commands can be done in several ways:

  • Initially, via a userdata script or userdata file specified in the host group or via the API/CLI create command after Slicer has been started.
  • Through SSH assuming direct network access is available to the VM via the LAN or a VPN.
  • Through Slicer's REST API - using your own client, the SDK or the slicer CLI.

The exec command allows you to run commands remotely on VMs. The first argument is the instance to run the commands on, -- optionally separates the slicer CLI options from the command to run on the VM.

# Run a command as root (default)
slicer vm exec vm-1 whoami
Slicer allows you to control the command execution context, including user permissions and working directory.

# Run command as non-root user (UID 1000)
slicer vm exec vm-1 --uid 1000 whoami

Change the working directory and run commands with arguments:

# Execute command in specific directory
slicer vm exec vm-1 --cwd /var/log -- ls -la

Control shell interpretation for complex commands:

# Use default bash shell for complex commands with pipes
slicer vm exec vm-1 -- "ls -l | sort"

# Use custom shell interpreter
slicer vm exec vm-1 --shell /bin/zsh -- "echo $SHELL"

# Execute directly without shell (faster for simple commands)
slicer vm exec vm-1 --shell "" whoami

Combine with local commands using pipes and STDIO:

# Pipe local file content to VM command
cat /etc/hostname | slicer vm exec vm-1 -- base64 --wrap 9999

Streaming vs buffered exec via the REST API

The /vm/{hostname}/exec REST endpoint ships two response shapes:

  • Streaming (default) — NDJSON frames as output arrives. Preferred for long-running commands where you want live stdout/stderr, or when you want to measure process-start latency separately from first-byte latency using the typed started frame.
  • Buffered — add buffered=true to get a single JSON document with stdout, stderr, and exit_code once the process exits. Preferred for short "run one thing, give me the result" calls; avoids the need for client-side NDJSON parsing.
# Streaming (default)
curl --unix-socket ~/slicer-mac/slicer.sock \
  -X POST "http://localhost/vm/vm-1/exec?cmd=uname&args=-a"

# Buffered
curl --unix-socket ~/slicer-mac/slicer.sock \
  -X POST "http://localhost/vm/vm-1/exec?buffered=true&cmd=uname&args=-a"
# → {"stdout":"Linux vm-1 ...\n","stderr":"","exit_code":0}

buffered=true does not accept stdin; use the streaming endpoint if you need to pipe stdin data.