Added more grep support (#29)

Added an `egrep` alias, as well as support for `-F`, `-E`, and `-e`
flags.

commands/grep.go

@@ -12,39 +12,62 @@ import (
 
 // Grep implements the POSIX grep command.
 //
-// https://pubs.opengroup.org/onlinepubs/9699919799.2018edition/
+// https://pubs.opengroup.org/onlinepubs/9699919799/utilities/grep.html
 func Grep(virtOS vos.VOS) int {
 	cmd := &SimpleCommand{
-		Use:   "grep [-iv] PATTERN [FILE]...",
+		Use:   "grep [-E|-F] [-iv] PATTERN [FILE]...",
 		Short: "Search files for text matching a pattern.",
 	}
 
 	invert := cmd.Flags().Bool('v', "Select lines not matching any of the specified patterns.")
 	ignoreCase := cmd.Flags().Bool('i', "Perform pattern matching in searches without regard to case.")
 	showLineNumbers := cmd.Flags().Bool('n', "Show line numbers.")
+	fixedStrings := cmd.Flags().Bool('F', "Treat patterns as strings rather than regular expressions.")
+	extendedRegex := cmd.Flags().Bool('E', "Use extended regular expressions.")
+
+	var patternList string
+	patternListFlag := cmd.Flags().Flag(&patternList, 'e', "Pattern to be used during the search for input.")
 
 	return cmd.Run(virtOS, func() int {
 		args := cmd.Flags().Args()
-		if len(args) == 0 {
+		// NOTE: Officially, the PATTERN argument supports multiple patterns delimited by newlines.
+		// It's a very rare case so we'll ignore it here.
+		var pattern string
+		switch {
+		case patternListFlag.Seen():
+			pattern = patternList
+
+		case len(args) == 0:
 			cmd.LogProgramError(virtOS, errors.New("missing argument PATTERN"))
 			return 1
+
+		default:
+			pattern = args[0]
+			args = args[1:]
+		}
+
+		switch {
+		case *extendedRegex || virtOS.Args()[0] == "egrep":
+			// Treat the pattern as regex.
+		case *fixedStrings:
+			pattern = regexp.QuoteMeta(pattern)
+		default:
+			// TODO: grep should treat the normal mode as "basic" regular expressions
+			// rather than extended: https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_03
 		}
 
-		// NOTE: Officially, the PATTERN argument supports multiple patterns delimited by newlines.
-		// It's a very rare case so we'll ignore it here.
-		pattern := args[0]
 		if *ignoreCase {
 			pattern = "(?i)" + pattern
 		}
+
 		regex, err := regexp.Compile(pattern)
 		if err != nil {
 			cmd.LogProgramError(virtOS, err)
 			return 2
 		}
 
-		files := args[1:]
-		showFileName := len(files) > 1
-		return cmd.RunEachFileOrStdin(virtOS, files, func(name string, fd io.Reader) error {
+		showFileName := len(args) > 1
+		return cmd.RunEachFileOrStdin(virtOS, args, func(name string, fd io.Reader) error {
 			w := virtOS.Stdout()
 
 			scanner := bufio.NewScanner(fd)
@@ -77,4 +100,5 @@ var _ vos.ProcessFunc = Grep
 
 func init() {
 	addBinCmd("grep", Grep)
+	addBinCmd("egrep", Grep)
 }