Added grep (#28)

Fixes #27.

Basic features of grep including line numbers, stdin, files, case
insensitivity, and inversion are supported.

Additionally, I've refactored base to support the common pattern of
running for multiple files or stdin if none are specified. This may
cause a refactor in `wc`, and `cat` -- possibly others.

commands/base.go

@@ -10,8 +10,8 @@ import (
 
 	"github.com/fatih/color"
 	fcolor "github.com/fatih/color"
-	getopt "github.com/pborman/getopt/v2"
 	"github.com/josephlewis42/honeyssh/core/vos"
+	getopt "github.com/pborman/getopt/v2"
 )
 
 // Always generate golden files on generate, a diff indicates a problem and
@@ -200,7 +200,7 @@ func (s *SimpleCommand) RunEachArg(virtOS vos.VOS, callback func(string) error)
 
 		for _, arg := range s.Flags().Args() {
 			if err := callback(arg); err != nil {
-				fmt.Fprintf(virtOS.Stderr(), "%s: %s\n", s.Flags().Program(), err.Error())
+				s.LogProgramError(virtOS, err)
 				anyErrored = true
 			}
 		}
@@ -216,13 +216,55 @@ func (s *SimpleCommand) RunEachArg(virtOS vos.VOS, callback func(string) error)
 func (s *SimpleCommand) RunE(virtOS vos.VOS, callback func() error) int {
 	return s.Run(virtOS, func() int {
 		if err := callback(); err != nil {
-			fmt.Fprintf(virtOS.Stderr(), "%s: %s\n", s.Flags().Program(), err.Error())
+			s.LogProgramError(virtOS, err)
 			return 1
 		}
 		return 0
 	})
 }
 
+// RunEachFileOrStdin runs the callback for every supplied arg, or stdin
+func (s *SimpleCommand) RunEachFileOrStdin(virtOS vos.VOS, files []string, callback func(name string, fd io.Reader) error) int {
+	return s.Run(virtOS, func() int {
+		anyErrored := false
+
+		openCallback := func(name string) error {
+			fd, err := virtOS.Open(name)
+			if err != nil {
+				return err
+			}
+
+			defer fd.Close()
+			return callback(name, fd)
+		}
+
+		for _, arg := range files {
+			if err := openCallback(arg); err != nil {
+				s.LogProgramError(virtOS, err)
+				anyErrored = true
+			}
+
+		}
+
+		if len(files) == 0 {
+			if err := callback("-", virtOS.Stdin()); err != nil {
+				s.LogProgramError(virtOS, err)
+				anyErrored = true
+			}
+		}
+
+		if anyErrored {
+			return 1
+		}
+		return 0
+	})
+}
+
+// Log a program error to stderr in the form "program name: error message"
+func (s *SimpleCommand) LogProgramError(virtOS vos.VOS, err error) {
+	fmt.Fprintf(virtOS.Stderr(), "%s: %s\n", s.Flags().Program(), err.Error())
+}
+
 const (
 	colorAlways = "always"
 	colorAuto   = "auto"

commands/grep.go

@@ -0,0 +1,80 @@
+package commands
+
+import (
+	"bufio"
+	"errors"
+	"fmt"
+	"io"
+	"regexp"
+
+	"github.com/josephlewis42/honeyssh/core/vos"
+)
+
+// Grep implements the POSIX grep command.
+//
+// https://pubs.opengroup.org/onlinepubs/9699919799.2018edition/
+func Grep(virtOS vos.VOS) int {
+	cmd := &SimpleCommand{
+		Use:   "grep [-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.")
+
+	return cmd.Run(virtOS, func() int {
+		args := cmd.Flags().Args()
+		if len(args) == 0 {
+			cmd.LogProgramError(virtOS, errors.New("missing argument PATTERN"))
+			return 1
+		}
+
+		// 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 {
+			w := virtOS.Stdout()
+
+			scanner := bufio.NewScanner(fd)
+			lineNo := 1
+			for scanner.Scan() {
+				line := scanner.Bytes()
+				lineMatches := regex.Match(line)
+
+				// Write output
+				if (lineMatches && !*invert) || (!lineMatches && *invert) {
+					if showFileName {
+						fmt.Fprintf(w, "%s:", name)
+					}
+
+					if *showLineNumbers {
+						fmt.Fprintf(w, "%d:", lineNo)
+					}
+
+					fmt.Fprintf(w, "%s\n", line)
+				}
+				lineNo++
+			}
+
+			return nil
+		})
+	})
+}
+
+var _ vos.ProcessFunc = Grep
+
+func init() {
+	addBinCmd("grep", Grep)
+}