diff --git a/src/proxy.go b/src/proxy.go
index d53805b1ac9..caddee4891f 100644
--- a/src/proxy.go
+++ b/src/proxy.go
@@ -9,6 +9,7 @@ import (
 	"os/exec"
 	"os/signal"
 	"path/filepath"
+	"regexp"
 	"strings"
 	"time"
 
@@ -32,7 +33,7 @@ func fifo(name string) (string, error) {
 	return output, nil
 }
 
-func runProxy(commandPrefix string, cmdBuilder func(temp string) *exec.Cmd, opts *Options, withExports bool) (int, error) {
+func runProxy(commandPrefix string, cmdBuilder func(temp string, needBash bool) (*exec.Cmd, error), opts *Options, withExports bool) (int, error) {
 	output, err := fifo("proxy-output")
 	if err != nil {
 		return ExitError, err
@@ -92,17 +93,28 @@ func runProxy(commandPrefix string, cmdBuilder func(temp string) *exec.Cmd, opts
 	// To ensure that the options are processed by a POSIX-compliant shell,
 	// we need to write the command to a temporary file and execute it with sh.
 	var exports []string
+	needBash := false
 	if withExports {
-		exports = os.Environ()
-		for idx, pairStr := range exports {
+		validIdentifier := regexp.MustCompile(`^[a-zA-Z_][a-zA-Z0-9_]*$`)
+		for _, pairStr := range os.Environ() {
 			pair := strings.SplitN(pairStr, "=", 2)
-			exports[idx] = fmt.Sprintf("export %s=%s", pair[0], escapeSingleQuote(pair[1]))
+			if validIdentifier.MatchString(pair[0]) {
+				exports = append(exports, fmt.Sprintf("export %s=%s", pair[0], escapeSingleQuote(pair[1])))
+			} else if strings.HasPrefix(pair[0], "BASH_FUNC_") && strings.HasSuffix(pair[0], "%%") {
+				name := pair[0][10 : len(pair[0])-2]
+				exports = append(exports, name+pair[1])
+				exports = append(exports, "export -f "+name)
+				needBash = true
+			}
 		}
 	}
 	temp := WriteTemporaryFile(append(exports, command), "\n")
 	defer os.Remove(temp)
 
-	cmd := cmdBuilder(temp)
+	cmd, err := cmdBuilder(temp, needBash)
+	if err != nil {
+		return ExitError, err
+	}
 	cmd.Stderr = os.Stderr
 	intChan := make(chan os.Signal, 1)
 	defer close(intChan)
diff --git a/src/proxy_unix.go b/src/proxy_unix.go
index 189d0e5694b..b67005342e8 100644
--- a/src/proxy_unix.go
+++ b/src/proxy_unix.go
@@ -9,7 +9,10 @@ import (
 	"golang.org/x/sys/unix"
 )
 
-func sh() (string, error) {
+func sh(bash bool) (string, error) {
+	if bash {
+		return "bash", nil
+	}
 	return "sh", nil
 }
 
diff --git a/src/proxy_windows.go b/src/proxy_windows.go
index a957da8a15e..2aa61ab77f3 100644
--- a/src/proxy_windows.go
+++ b/src/proxy_windows.go
@@ -13,12 +13,16 @@ import (
 
 var shPath atomic.Value
 
-func sh() (string, error) {
+func sh(bash bool) (string, error) {
 	if cached := shPath.Load(); cached != nil {
 		return cached.(string), nil
 	}
 
-	cmd := exec.Command("cygpath", "-w", "/usr/bin/sh")
+	name := "sh"
+	if bash {
+		name = "bash"
+	}
+	cmd := exec.Command("cygpath", "-w", "/usr/bin/"+name)
 	bytes, err := cmd.Output()
 	if err != nil {
 		return "", err
@@ -31,7 +35,7 @@ func sh() (string, error) {
 
 func mkfifo(path string, mode uint32) (string, error) {
 	m := strconv.FormatUint(uint64(mode), 8)
-	sh, err := sh()
+	sh, err := sh(false)
 	if err != nil {
 		return path, err
 	}
@@ -43,7 +47,7 @@ func mkfifo(path string, mode uint32) (string, error) {
 }
 
 func withOutputPipe(output string, task func(io.ReadCloser)) error {
-	sh, err := sh()
+	sh, err := sh(false)
 	if err != nil {
 		return err
 	}
@@ -62,7 +66,7 @@ func withOutputPipe(output string, task func(io.ReadCloser)) error {
 }
 
 func withInputPipe(input string, task func(io.WriteCloser)) error {
-	sh, err := sh()
+	sh, err := sh(false)
 	if err != nil {
 		return err
 	}
diff --git a/src/tmux.go b/src/tmux.go
index 246222f928d..b2315dcd2c3 100644
--- a/src/tmux.go
+++ b/src/tmux.go
@@ -49,9 +49,12 @@ func runTmux(args []string, opts *Options) (int, error) {
 	tmuxArgs = append(tmuxArgs, "-w"+opts.Tmux.width.String())
 	tmuxArgs = append(tmuxArgs, "-h"+opts.Tmux.height.String())
 
-	return runProxy(argStr, func(temp string) *exec.Cmd {
-		sh, _ := sh()
+	return runProxy(argStr, func(temp string, needBash bool) (*exec.Cmd, error) {
+		sh, err := sh(needBash)
+		if err != nil {
+			return nil, err
+		}
 		tmuxArgs = append(tmuxArgs, sh, temp)
-		return exec.Command("tmux", tmuxArgs...)
+		return exec.Command("tmux", tmuxArgs...), nil
 	}, opts, true)
 }
diff --git a/src/winpty_windows.go b/src/winpty_windows.go
index 78020d73810..aba02ce2210 100644
--- a/src/winpty_windows.go
+++ b/src/winpty_windows.go
@@ -44,11 +44,6 @@ func needWinpty(opts *Options) bool {
 }
 
 func runWinpty(args []string, opts *Options) (int, error) {
-	sh, err := sh()
-	if err != nil {
-		return ExitError, err
-	}
-
 	argStr := escapeSingleQuote(args[0])
 	for _, arg := range args[1:] {
 		argStr += " " + escapeSingleQuote(arg)
@@ -56,20 +51,30 @@ func runWinpty(args []string, opts *Options) (int, error) {
 	argStr += ` --no-winpty`
 
 	if isMintty345() {
-		return runProxy(argStr, func(temp string) *exec.Cmd {
+		return runProxy(argStr, func(temp string, needBash bool) (*exec.Cmd, error) {
+			sh, err := sh(needBash)
+			if err != nil {
+				return nil, err
+			}
+
 			cmd := exec.Command(sh, temp)
 			cmd.Env = append(os.Environ(), "MSYS=enable_pcon")
 			cmd.Stdin = os.Stdin
 			cmd.Stdout = os.Stdout
 			cmd.Stderr = os.Stderr
-			return cmd
+			return cmd, nil
 		}, opts, false)
 	}
 
-	return runProxy(argStr, func(temp string) *exec.Cmd {
+	return runProxy(argStr, func(temp string, needBash bool) (*exec.Cmd, error) {
+		sh, err := sh(needBash)
+		if err != nil {
+			return nil, err
+		}
+
 		cmd := exec.Command(sh, "-c", fmt.Sprintf(`winpty < /dev/tty > /dev/tty -- sh %q`, temp))
 		cmd.Stdout = os.Stdout
 		cmd.Stderr = os.Stderr
-		return cmd
+		return cmd, nil
 	}, opts, false)
 }