Add POSIX shell support

Any users of a shell that adheres to the POSIX standard should now be
supported. Shells that were tested while this feature was in development
include `mrsh`, `dash`, busybox `ash`, and `bash --posix`.

The hook works by defining a `_zoxide_hook` function and adding it to
the shell's `PS1` (causing it to be evaluated every time the prompt is
redrawn).

The PWD hook has been removed, because it is impossible to implement in
a POSIX-compliant and satisfactory way.

README.md

@@ -15,6 +15,7 @@ A cd command that learns your habits
     - [zsh](#zsh)
     - [bash](#bash)
     - [fish](#fish)
+    - [POSIX](#posix)
 - [Configuration](#configuration)
   - [`init` flags](#init-flags)
   - [Environment variables](#environment-variables)
@@ -89,6 +90,20 @@ Add the following line to your `~/.config/fish/config.fish`:
 zoxide init fish | source
 ```
 
+#### POSIX
+
+Add the following line to your shell's configuration file (or, run it manually):
+
+``` sh
+eval "$(zoxide init posix)"
+```
+
+NOTE: If you modify your `PS1` at any point, you may need to re-run the above command. This is due
+to the fact that we store our hook in `PS1`, in order to be evaluated every time the prompt is
+displayed.
+
+NOTE: There is no PWD hook provided for POSIX shells.
+
 ## Configuration
 
 ### `init` flags

src/subcommand/init.rs

@@ -29,6 +29,7 @@ impl Init {
         let config = match self.shell {
             Shell::bash => BASH_CONFIG,
             Shell::fish => FISH_CONFIG,
+            Shell::posix => POSIX_CONFIG,
             Shell::zsh => ZSH_CONFIG,
         };
 
@@ -43,7 +44,11 @@ impl Init {
         match self.hook {
             Hook::none => (),
             Hook::prompt => writeln!(handle, "{}", config.hook.prompt).unwrap(),
-            Hook::pwd => writeln!(handle, "{}", config.hook.pwd).unwrap(),
+            Hook::pwd => {
+                if let Some(pwd_hook) = config.hook.pwd {
+                    writeln!(handle, "{}", pwd_hook).unwrap();
+                }
+            }
         };
     }
 }
@@ -54,6 +59,7 @@ arg_enum! {
     enum Shell {
         bash,
         fish,
+        posix,
         zsh,
     }
 }
@@ -73,7 +79,7 @@ const BASH_CONFIG: ShellConfig = ShellConfig {
     alias: BASH_ALIAS,
     hook: HookConfig {
         prompt: BASH_HOOK_PROMPT,
-        pwd: BASH_HOOK_PWD,
+        pwd: Some(BASH_HOOK_PWD),
     },
 };
 
@@ -82,7 +88,16 @@ const FISH_CONFIG: ShellConfig = ShellConfig {
     alias: FISH_ALIAS,
     hook: HookConfig {
         prompt: FISH_HOOK_PROMPT,
-        pwd: FISH_HOOK_PWD,
+        pwd: Some(FISH_HOOK_PWD),
+    },
+};
+
+const POSIX_CONFIG: ShellConfig = ShellConfig {
+    z: POSIX_Z,
+    alias: POSIX_ALIAS,
+    hook: HookConfig {
+        prompt: POSIX_HOOK_PROMPT,
+        pwd: None,
     },
 };
 
@@ -91,7 +106,7 @@ const ZSH_CONFIG: ShellConfig = ShellConfig {
     alias: ZSH_ALIAS,
     hook: HookConfig {
         prompt: ZSH_HOOK_PROMPT,
-        pwd: ZSH_HOOK_PWD,
+        pwd: Some(ZSH_HOOK_PWD),
     },
 };
 
@@ -103,7 +118,7 @@ struct ShellConfig {
 
 struct HookConfig {
     prompt: &'static str,
-    pwd: &'static str,
+    pwd: Option<&'static str>,
 }
 
 const BASH_Z: &str = r#"
@@ -119,12 +134,17 @@ z() {
     if [ "$#" -eq 0 ]; then
         _z_cd ~ || return "$?"
     elif [ "$#" -eq 1 ] && [ "$1" = '-' ]; then
-        _z_cd ~- || return "$?"
+        if [ -n "$OLDPWD" ]; then
+            _z_cd "$OLDPWD" || return "$?"
+        else
+            echo "zoxide: \$OLDPWD is not set"
+            return 1
+        fi
     else
-        result="$(zoxide query $@)" || return "$?"
+        result="$(zoxide query "$@")" || return "$?"
         case "$result" in
             "query: "*)
-                _z_cd "${result:7}" || return "$?"
+                _z_cd "${result#query: }" || return "$?"
                 ;;
             *)
                 if [ -n "$result" ]; then
@@ -177,6 +197,8 @@ function z
 end
 "#;
 
+const POSIX_Z: &str = BASH_Z;
+
 const ZSH_Z: &str = BASH_Z;
 
 const BASH_ALIAS: &str = r#"
@@ -193,6 +215,8 @@ abbr -a zq 'zoxide query'
 abbr -a zr 'zoxide remove'
 "#;
 
+const POSIX_ALIAS: &str = BASH_ALIAS;
+
 const ZSH_ALIAS: &str = BASH_ALIAS;
 
 const BASH_HOOK_PROMPT: &str = r#"
@@ -212,6 +236,17 @@ function _zoxide_hook --on-event fish_prompt
 end
 "#;
 
+const POSIX_HOOK_PROMPT: &str = r#"
+_zoxide_hook() {
+    zoxide add
+}
+
+case "$PS1" in
+    *\$\(_zoxide_hook\)*) ;;
+    *) PS1="\$(_zoxide_hook)${PS1}" ;;
+esac
+"#;
+
 const ZSH_HOOK_PROMPT: &str = r#"
 _zoxide_hook() {
     zoxide add