What causes "destination can not be empty" in docker/cli?

Provide a concrete destination path or container:path, or '-' to stream a tar to stdout.

destination can not be empty

Error message

destination can not be empty

What it means

In newCopyCommand RunE at line 143-144, after confirming args[0] is non-empty, the code checks args[1] and returns errors.New("destination can not be empty") if the second positional arg is the empty string. The copy must have a concrete destination (local path or container:path, or '-' to stream to stdout).

Solutions

Provide a concrete destination path or container:path, or '-' to stream a tar to stdout.
Guard your shell variable: `${DEST:?destination required}`.
Double-check quoting/expansion when building args dynamically.

Example fix

# before
DEST=""
docker cp web:/tmp/out "$DEST"
# after
DEST=./out.txt
docker cp web:/tmp/out "$DEST"
# or stream to stdout
docker cp web:/tmp/out -

Defensive patterns

Strategy: validation

Validate before calling

// Guard shell variable expansion:
// ${DEST:?destination required} aborts if DEST is unset/empty.
// In Go:
if dest == "" { return errors.New("destination can not be empty") }

Try / catch

if err := cmd.Execute(); err != nil {
    if strings.Contains(err.Error(), "destination can not be empty") {
        // prompt for destination / fix argv
    }
}

Prevention

Quote and check shell variables before `docker cp` (`${DEST:?}`).
Use '-' explicitly to stream a tar to stdout.
Validate argv in wrappers before invoking docker.

When it happens

Trigger: Running `docker cp web:/tmp/file ""`, `docker cp ./file ""`, or any invocation where the destination positional argument is an empty string (unset/empty shell variable).

Common situations: Unset destination shell variables; programmatic argv construction leaving args[1] as ""; intending to stream to stdout but forgetting the '-' character.

Related errors

AI-assisted analysis of docker/cli@4f84911bfe (2026-08-07). Data as JSON: /api/errors/fa15014d010a0590. Report an issue: GitHub.

Appendix: source

Thrown at cli/command/container/cp.go:144

	var opts copyOptions

	cmd := &cobra.Command{
		Use: `cp [OPTIONS] CONTAINER:SRC_PATH DEST_PATH|-
	docker cp [OPTIONS] SRC_PATH|- CONTAINER:DEST_PATH`,
		Short: "Copy files/folders between a container and the local filesystem",
		Long: `Copy files/folders between a container and the local filesystem

Use '-' as the source to read a tar archive from stdin
and extract it to a directory destination in a container.
Use '-' as the destination to stream a tar archive of a
container source to stdout.`,
		Args: cli.ExactArgs(2),
		RunE: func(cmd *cobra.Command, args []string) error {
			if args[0] == "" {
				return errors.New("source can not be empty")
			}
			if args[1] == "" {
				return errors.New("destination can not be empty")
			}
			opts.source = args[0]
			opts.destination = args[1]
			if !cmd.Flag("quiet").Changed {
				// User did not specify "quiet" flag; suppress output if no terminal is attached
				opts.quiet = !dockerCLI.Out().IsTerminal()
			}
			return runCopy(cmd.Context(), dockerCLI, opts)
		},
		Annotations: map[string]string{
			"aliases": "docker container cp, docker cp",
		},
		DisableFlagsInUseLine: true,
	}

	flags := cmd.Flags()
	flags.BoolVarP(&opts.followLink, "follow-link", "L", false, "Always follow symlinks in SRC_PATH")
	flags.BoolVarP(&opts.copyUIDGID, "archive", "a", false, "Archive mode (copy all uid/gid information)")

View on GitHub (pinned to 4f84911bfe)