expand,interp: trim whitespace in atoi

Fixes #928.

expand/arith.go

@@ -6,6 +6,7 @@ package expand
 import (
 	"fmt"
 	"strconv"
+	"strings"
 
 	"mvdan.cc/sh/v3/syntax"
 )
@@ -105,9 +106,9 @@ func oneIf(b bool) int {
 	return 0
 }
 
-// atoi is just a shorthand for strconv.Atoi that ignores the error,
-// just like shells do.
+// atoi is like strconv.Atoi, but it ignores errors and trims whitespace.
 func atoi(s string) int {
+	s = strings.TrimSpace(s)
 	n, _ := strconv.Atoi(s)
 	return n
 }

interp/builtin.go

@@ -32,16 +32,18 @@ func isBuiltin(name string) bool {
 	return false
 }
 
+// TODO: oneIf and atoi are duplicated in the expand package.
+
 func oneIf(b bool) int {
 	if b {
 		return 1
 	}
 	return 0
 }
 
-// atoi is just a shorthand for strconv.Atoi that ignores the error,
-// just like shells do.
+// atoi is like strconv.Atoi, but it ignores errors and trims whitespace.
 func atoi(s string) int {
+	s = strings.TrimSpace(s)
 	n, _ := strconv.Atoi(s)
 	return n
 }

interp/interp_test.go

@@ -1412,6 +1412,10 @@ var runTests = []runTest{
 		"[[ 3 -lt 4 ]]",
 		"",
 	},
+	{
+		"[[ ' 3' -lt '4 ' ]]",
+		"",
+	},
 	{
 		"[[ 3 -gt 4 ]]",
 		"exit status 1",
@@ -1637,6 +1641,7 @@ var runTests = []runTest{
 	{"[ 0 -gt 1 -o 1 -gt 0 ]", ""},
 	{"[ 3 -gt 4 ]", "exit status 1"},
 	{"[ 3 -lt 4 ]", ""},
+	{"[ ' 3' -lt '4 ' ]", ""},
 	{
 		"[ -e a ] && echo x; >a; [ -e a ] && echo y",
 		"y\n",
@@ -1826,6 +1831,14 @@ var runTests = []runTest{
 		"let x=3; let 3%0; ((3%0)); echo $((x%y)); let x%=0",
 		"division by zero\ndivision by zero\ndivision by zero\ndivision by zero\nexit status 1 #JUSTERR",
 	},
+	{
+		"let x=' 3'; echo $x",
+		"3\n",
+	},
+	{
+		"x=' 3'; let x++; echo \"$x\"",
+		"4\n",
+	},
 
 	// set/shift
 	{