slimtoolkit/slim · error

bad instruction prefix

Error message

bad instruction prefix

What it means

ErrBadInstPrefix is returned by the dockerfile reverse-engineering package when a parsed instruction line does not start with a recognized Dockerfile instruction prefix. The reverse parser maps image history/lines back to Dockerfile instructions and cannot classify a line whose prefix it does not know. It signals unexpected or malformed line content rather than caller misuse.

Source

Thrown at pkg/docker/dockerfile/reverse/reverse.go:21

import (
	"bytes"
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"sort"
	"strconv"
	"strings"
	"time"

	"github.com/dustin/go-humanize"
	docker "github.com/fsouza/go-dockerclient"
	"github.com/google/shlex"
	log "github.com/sirupsen/logrus"
)

var (
	ErrBadInstPrefix = errors.New("bad instruction prefix")
)

// Dockerfile represents the reverse engineered Dockerfile info
type Dockerfile struct {
	Lines                    []string             `json:"lines,omitempty"`
	Maintainers              []string             `json:"maintainers,omitempty"`
	AllUsers                 []string             `json:"all_users,omitempty"`
	ExeUser                  string               `json:"exe_user,omitempty"`
	ExposedPorts             []string             `json:"exposed_ports,omitempty"`
	ImageStack               []*ImageInfo         `json:"image_stack"`
	AllInstructions          []*InstructionInfo   `json:"all_instructions"`
	InstructionGroups        [][]*InstructionInfo `json:"instruction_groups"`
	InstructionGroupsReverse [][]*InstructionInfo `json:"instruction_groups_reverse"`
	HasOnbuild               bool                 `json:"has_onbuild"`
}

type ImageInfo struct {
	IsTopImage   bool               `json:"is_top_image"`

View on GitHub (pinned to 81940d17fa)

Solutions

1. Inspect the source image's history (docker history --no-trunc) for lines with unexpected first tokens.
2. Update slimtoolkit to a version whose reverse parser recognizes the instruction prefix in question.
3. Skip or sanitize unrecognized history lines before passing them to the reverse package.
4. If the line is a builder comment or metadata artifact, strip it prior to reverse engineering.

firstToken := strings.Fields(line)[0]
known := map[string]bool{"FROM":true,"RUN":true,"CMD":true,"LABEL":true,"MAINTAINER":true,"EXPOSE":true,"ENV":true,"ADD":true,"COPY":true,"ENTRYPOINT":true,"VOLUME":true,"USER":true,"WORKDIR":true,"ARG":true,"ONBUILD":true,"STOPSIGNAL":true,"HEALTHCHECK":true,"SHELL":true}
if !known[strings.ToUpper(firstToken)] { /* skip or log line */ }

df, err := reverse.FromImage(...)
if errors.Is(err, reverse.ErrBadInstPrefix) {
    log.Warnf("skipping unrecognized instruction line: %v", err)
    // proceed with partial result or sanitize input and retry
}

When it happens

Trigger: Reverse-engineering a Dockerfile from image history where a history line's first token is not a known instruction keyword (FROM, RUN, MAINTAINER, etc.).

Common situations: Images built by non-standard builders or with unusual metadata comments in history; older/newer Docker instruction keywords not covered by the reverse parser's prefix table.

Related errors

• when using JSON array syntax, arrays must be comprised of st
• unknown instruction
• only one escape parser directive can be used
• invalid Dockerfile
• invalid dockerfile reference - %s

AI-assisted analysis of slimtoolkit/slim@81940d17fa (2026-08-31). Data as JSON: /api/errors/6b6c653a34edd69d. Report an issue: GitHub.