undocker/main.go

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// Package main is a simple command-line application on top of rootfs.Flatten.
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package main
import (
"flag"
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"fmt"
"io"
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"os"
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"path/filepath"
"runtime"
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"git.sr.ht/~motiejus/undocker/rootfs"
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)
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var Version = "unknown"
var VersionHash = "unknown"
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const _usage = `Usage:
%s [OPTION]... <infile> <outfile>
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Flatten a Docker container image to a root file system.
Arguments:
<infile> Input Docker container. Tarball.
<outfile> Output tarball, the root file system. '-' is stdout.
Options:
--prefix=<prefix> prefix all destination files with a given string.
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undocker %s (%s)
Built with %s
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`
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func usage(pre string, out io.Writer) {
fmt.Fprintf(out, pre+_usage,
filepath.Base(os.Args[0]),
Version,
VersionHash,
runtime.Version(),
)
}
func usageErr(pre string) {
usage(pre, os.Stderr)
os.Exit(2)
}
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func main() {
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runtime.GOMAXPROCS(1) // no need to create that many threads
var filePrefix string
fs := flag.NewFlagSet("undocker", flag.ExitOnError)
fs.Usage = func() { usageErr("") }
fs.StringVar(&filePrefix, "prefix", "", "prefix files in the tarball")
if len(os.Args) == 1 {
usageErr("")
}
_ = fs.Parse(os.Args[1:]) // ExitOnError captures it
args := fs.Args()
if len(args) != 2 {
usageErr("invalid number of arguments\n")
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}
c := &command{flattener: rootfs.Flatten, Stdout: os.Stdout}
if err := c.execute(args[0], args[1], filePrefix); err != nil {
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fmt.Printf("Error: %v\n", err)
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os.Exit(1)
}
os.Exit(0)
}
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type command struct {
flattener func(io.ReadSeeker, io.Writer, ...rootfs.Option) error
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Stdout io.Writer
}
func (c *command) execute(infile, outfile, filePrefix string) (_err error) {
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rd, err := os.Open(infile)
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if err != nil {
return err
}
defer func() {
err := rd.Close()
if _err == nil {
_err = err
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}
}()
var out io.Writer
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if outfile == "-" {
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out = c.Stdout
} else {
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outf, err := os.Create(outfile)
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if err != nil {
return fmt.Errorf("create: %w", err)
}
defer func() {
err := outf.Close()
if _err != nil {
os.Remove(outfile)
} else {
_err = err
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}
}()
out = outf
}
return c.flattener(rd, out, rootfs.WithFilePrefix(filePrefix))
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}