script: Implement simple "sed" command

The simple 'replace' command falls short in situations
where the input text isn't fully known, but still needs
to be sanitized in some way. Instead of having to use e.g.
the system 'sed' command, implement a very simple built-in
version.

Signed-off-by: Jussi Maki <[email protected]>

script/cmds.go

@@ -44,6 +44,7 @@ func DefaultCmds() map[string]Cmd {
 		"mv":      Mv(),
 		"rm":      Rm(),
 		"replace": Replace(),
+		"sed":     Sed(),
 		"sleep":   Sleep(),
 		"stderr":  Stderr(),
 		"stdout":  Stdout(),
@@ -882,6 +883,45 @@ func Replace() Cmd {
 		})
 }
 
+// Sed implements a simple regexp replacement of text in a file.
+func Sed() Cmd {
+	return Command(
+		CmdUsage{
+			Summary: "substitute strings in a file with a regexp",
+			Args:    "regexp replacement file",
+			Detail: []string{
+				"A simple sed-like command for replacing text matching regular expressions",
+				"in a file.",
+				"",
+				"Implemented using regexp.ReplaceAll, see its docs for what is supported:",
+				"https://pkg.go.dev/regexp#Regexp.ReplaceAll",
+			},
+			RegexpArgs: firstNonFlag,
+		},
+		func(s *State, args ...string) (WaitFunc, error) {
+			if len(args) != 3 {
+				return nil, ErrUsage
+			}
+
+			re, err := regexp.Compile(args[0])
+			if err != nil {
+				return nil, err
+			}
+			replacement := args[1]
+			file := s.Path(args[2])
+
+			data, err := os.ReadFile(file)
+			if err != nil {
+				return nil, err
+			}
+			lines := strings.Split(string(data), "\n")
+			for i, line := range lines {
+				lines[i] = re.ReplaceAllString(line, replacement)
+			}
+			return nil, os.WriteFile(file, []byte(strings.Join(lines, "\n")), 0666)
+		})
+}
+
 // Rm removes a file or directory.
 //
 // If a directory, Rm also recursively removes that directory's

script/scripttest/testdata/sed.txt

@@ -0,0 +1,20 @@
+# Tests for the sed command
+
+sed '' '' input.txt
+sed notfound unexpected input.txt
+sed \d+ N input.txt
+sed [uz] '' input.txt
+sed a(x*)b 'a${1}c' input.txt
+sed ^(s+)$ '${1}${1}' input.txt
+cmp input.txt expected.txt
+
+-- input.txt --
+foo123bar
+quuxbaz123
+axxxxxxb
+sss
+-- expected.txt --
+fooNbar
+qxbaN
+axxxxxxc
+ssssss