export OLDPWD variable as the initial directory

cc #601

doc.go

@@ -144,6 +144,7 @@ The following environment variables are exported for shell commands:
     fx
     id
     PWD
+    OLDPWD
     LF_LEVEL
     OPENER
     EDITOR
@@ -716,6 +717,10 @@ Id of the running client.
 
 Present working directory.
 
+    OLDPWD
+
+Initial working directory.
+
     LF_LEVEL
 
 The value of this variable is set to the current nesting level when you run lf from a shell spawned inside lf.

lf.1

@@ -164,6 +164,7 @@ The following environment variables are exported for shell commands:
     fx
     id
     PWD
+    OLDPWD
     LF_LEVEL
     OPENER
     EDITOR
@@ -868,6 +869,12 @@ Id of the running client.
 .PP
 Present working directory.
 .PP
+.EX
+    OLDPWD
+.EE
+.PP
+Initial working directory.
+.PP
 .EX
     LF_LEVEL
 .EE

main.go

@@ -65,6 +65,12 @@ func exportEnvVars() {
 	os.Setenv("PAGER", envPager)
 	os.Setenv("SHELL", envShell)
 
+	dir, err := os.Getwd()
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "%s\n", err)
+	}
+	os.Setenv("OLDPWD", dir)
+
 	level, err := strconv.Atoi(envLevel)
 	if err != nil {
 		log.Printf("reading lf level: %s", err)