Recognize 'busybox' in --shell and directives. Add to doc texts.

shellcheck.1.md

@@ -85,7 +85,8 @@ not warn at all, as `ksh` supports decimals in arithmetic contexts.
 
 **-s**\ *shell*,\ **--shell=***shell*
 
-:   Specify Bourne shell dialect. Valid values are *sh*, *bash*, *dash* and *ksh*.
+:   Specify Bourne shell dialect. Valid values are *sh*, *bash*, *dash*, *ksh*,
+    and *busybox*.
     The default is to deduce the shell from the file's `shell` directive,
     shebang, or `.bash/.bats/.dash/.ksh` extension, in that order. *sh* refers to
     POSIX `sh` (not the system's), and will warn of portability issues.

shellcheck.hs

@@ -115,7 +115,7 @@ options = [
         "Specify path when looking for sourced files (\"SCRIPTDIR\" for script's dir)",
     Option "s" ["shell"]
         (ReqArg (Flag "shell") "SHELLNAME")
-        "Specify dialect (sh, bash, dash, ksh)",
+        "Specify dialect (sh, bash, dash, ksh, busybox)",
     Option "S" ["severity"]
         (ReqArg (Flag "severity") "SEVERITY")
         "Minimum severity of errors to consider (error, warning, info, style)",

src/ShellCheck/Checks/ShellSupport.hs

@@ -211,6 +211,8 @@ prop_checkBashisms116 = verify checkBashisms "#!/bin/busybox sh\nx='test'\n${x[1
 prop_checkBashisms117 = verify checkBashisms "#!/bin/busybox sh\nx='test'\n${!x[@]}" -- SC3055
 prop_checkBashisms118 = verify checkBashisms "#!/bin/busybox sh\nxyz=1\n${!x*}" -- SC3056
 prop_checkBashisms119 = verify checkBashisms "#!/bin/busybox sh\nx='test'\n${x^^[t]}" -- SC3059
+prop_checkBashisms120 = verify checkBashisms "#!/bin/sh\n[ x == y ]"
+prop_checkBashisms121 = verifyNot checkBashisms "#!/bin/sh\n# shellcheck shell=busybox\n[ x == y ]"
 checkBashisms = ForShell [Sh, Dash, BusyboxSh] $ \t -> do
     params <- ask
     kludge params t

src/ShellCheck/Data.hs

@@ -156,6 +156,7 @@ shellForExecutable name =
         "sh"    -> return Sh
         "bash"  -> return Bash
         "bats"  -> return Bash
+        "busybox"  -> return BusyboxSh -- Used for directives and --shell=busybox
         "busybox sh"  -> return BusyboxSh
         "busybox ash"  -> return BusyboxSh
         "dash"  -> return Dash