File: catl.go
   1 /*
   2 The MIT License (MIT)
   3 
   4 Copyright (c) 2026 pacman64
   5 
   6 Permission is hereby granted, free of charge, to any person obtaining a copy of
   7 this software and associated documentation files (the "Software"), to deal
   8 in the Software without restriction, including without limitation the rights to
   9 use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
  10 of the Software, and to permit persons to whom the Software is furnished to do
  11 so, subject to the following conditions:
  12 
  13 The above copyright notice and this permission notice shall be included in all
  14 copies or substantial portions of the Software.
  15 
  16 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  17 IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  18 FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  19 AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  20 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  21 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  22 SOFTWARE.
  23 */
  24 
  25 /*
  26 To compile a smaller-sized command-line app, you can use the `go` command as
  27 follows:
  28 
  29 go build -ldflags "-s -w" -trimpath catl.go
  30 */
  31 
  32 package main
  33 
  34 import (
  35     "bufio"
  36     "errors"
  37     "io"
  38     "os"
  39 )
  40 
  41 // Note: the code is avoiding using the fmt package to save hundreds of
  42 // kilobytes on the resulting executable, which is a noticeable difference.
  43 
  44 const info = `
  45 catl [options...] [file...]
  46 
  47 
  48 Unlike "cat", conCATenate Lines ensures lines across inputs are never joined
  49 by accident, when an input's last line doesn't end with a line-feed.
  50 
  51 Input is assumed to be UTF-8, and all CRLF byte-pairs are turned into line
  52 feeds. Leading BOM (byte-order marks) on first lines are also ignored.
  53 
  54 All (optional) leading options start with either single or double-dash:
  55 
  56     -h          show this help message
  57     -help       show this help message
  58 `
  59 
  60 // errNoMoreOutput is a dummy error whose message is ignored, and which
  61 // causes the app to quit immediately and successfully
  62 var errNoMoreOutput = errors.New(`no more output`)
  63 
  64 func main() {
  65     buffered := false
  66     args := os.Args[1:]
  67 
  68     if len(args) > 0 {
  69         switch args[0] {
  70         case `-b`, `--b`, `-buffered`, `--buffered`:
  71             buffered = true
  72             args = args[1:]
  73 
  74         case `-h`, `--h`, `-help`, `--help`:
  75             os.Stdout.WriteString(info[1:])
  76             return
  77         }
  78     }
  79 
  80     if len(args) > 0 && args[0] == `--` {
  81         args = args[1:]
  82     }
  83 
  84     liveLines := !buffered
  85     if !buffered {
  86         if _, err := os.Stdout.Seek(0, io.SeekCurrent); err == nil {
  87             liveLines = false
  88         }
  89     }
  90 
  91     if err := run(os.Stdout, args, liveLines); isActualError(err) {
  92         os.Stderr.WriteString(err.Error())
  93         os.Stderr.WriteString("\n")
  94         os.Exit(1)
  95     }
  96 }
  97 
  98 func run(w io.Writer, args []string, live bool) error {
  99     bw := bufio.NewWriter(w)
 100     defer bw.Flush()
 101 
 102     if len(args) == 0 {
 103         return catl(bw, os.Stdin, live)
 104     }
 105 
 106     for _, name := range args {
 107         if err := handleFile(bw, name, live); err != nil {
 108             return err
 109         }
 110     }
 111     return nil
 112 }
 113 
 114 func handleFile(w *bufio.Writer, name string, live bool) error {
 115     if name == `` || name == `-` {
 116         return catl(w, os.Stdin, live)
 117     }
 118 
 119     f, err := os.Open(name)
 120     if err != nil {
 121         return errors.New(`can't read from file named "` + name + `"`)
 122     }
 123     defer f.Close()
 124 
 125     return catl(w, f, live)
 126 }
 127 
 128 // isActualError is to figure out whether not to ignore an error, and thus
 129 // show it as an error message
 130 func isActualError(err error) bool {
 131     return err != nil && err != errNoMoreOutput
 132 }
 133 
 134 func catl(w *bufio.Writer, r io.Reader, live bool) error {
 135     const gb = 1024 * 1024 * 1024
 136     sc := bufio.NewScanner(r)
 137     sc.Buffer(nil, 8*gb)
 138 
 139     for i := 0; sc.Scan(); i++ {
 140         s := sc.Bytes()
 141         if i == 0 && len(s) > 2 && s[0] == 0xef && s[1] == 0xbb && s[2] == 0xbf {
 142             s = s[3:]
 143         }
 144 
 145         w.Write(s)
 146         if w.WriteByte('\n') != nil {
 147             return errNoMoreOutput
 148         }
 149 
 150         if !live {
 151             continue
 152         }
 153 
 154         if err := w.Flush(); err != nil {
 155             return errNoMoreOutput
 156         }
 157     }
 158 
 159     return sc.Err()
 160 }