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

Provide a non-empty source path or container:path spec: `docker cp ./file web:/tmp`.

source can not be empty

Error message

source can not be empty

What it means

In newCopyCommand RunE (cli/command/container/cp.go:139), after the Args validator (cli.ExactArgs(2)) ensures two args exist, the code explicitly checks args[0] at line 140-141 and returns errors.New("source can not be empty") if the first positional arg is the empty string. An empty source path has no meaning for the copy operation.

Solutions

Provide a non-empty source path or container:path spec: `docker cp ./file web:/tmp`.
Quote-check your shell variables: ensure `$SRC` is set and non-empty before the command (e.g. `${SRC:?source required}`).
If streaming a tar via stdin, use '-' as the source, not the empty string.

Example fix

# before
SRC=""
docker cp "$SRC" web:/tmp
# after
SRC=./file.txt
docker cp "$SRC" web:/tmp

Defensive patterns

Strategy: validation

Validate before calling

// Guard shell variable expansion:
// ${SRC:?source required} aborts if SRC is unset/empty.
// In Go, before building argv:
if source == "" { return errors.New("source can not be empty") }

Try / catch

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

Prevention

Quote and check shell variables before `docker cp` (`${SRC:?}`).
Use '-' explicitly for stdin tar streaming, never the empty string.
Validate argv in wrappers before invoking docker.

When it happens

Trigger: Running `docker cp "" dest`, `docker cp "" CONTAINER:/path`, or any invocation where the first positional argument resolves to an empty string (e.g. an unexpanded/empty shell variable like `docker cp "$EMPTY" web:/tmp`).

Common situations: Unset shell variables expanded to empty; quoting bugs; programmatic argv construction leaving args[0] as ""; `docker cp '' web:/x`.

Related errors

AI-assisted analysis of docker/cli@4f84911bfe (2026-08-07). Data as JSON: /api/errors/644e8944f0133c3c. Report an issue: GitHub.

Appendix: source

Thrown at cli/command/container/cp.go:141

// newCopyCommand creates a new `docker cp` command
func newCopyCommand(dockerCLI command.Cli) *cobra.Command {
	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,
	}

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