Add Sys.which(program_name::AbstractString)

This acts as a julia-native version of the `which` command found on most
*nix systems; it searches the path for an executable of the given name,
returning the absolute path if it exists, and throwing an
`ArgumentError()` if it does not.

base/sysinfo.jl

@@ -23,7 +23,8 @@ export BINDIR,
        isbsd,
        islinux,
        isunix,
-       iswindows
+       iswindows,
+       which
 
 import ..Base: show
 
@@ -311,6 +312,7 @@ if iswindows()
 else
     windows_version() = v"0.0"
 end
+
 """
     Sys.windows_version()
 
@@ -321,4 +323,66 @@ windows_version
 
 const WINDOWS_VISTA_VER = v"6.0"
 
+"""
+    Sys.which(program_name::AbstractString)
+
+Given a program name, searches the current `PATH` to find the first binary with
+the proper executable permissions that can be run, and returns the absolute
+path. Raises `ArgumentError` if no such program is available.  If a relative or
+absolute path is passed in for `program_name`, that exact path is tested for
+executable permissions only, no searching of `PATH` is performed.
+"""
+function which(program_name::AbstractString)
+    # We use `access()` and `X_OK` to determine if a given path is executable
+    # by the current user.  `X_OK` comes from `unistd.h`.
+    X_OK = 1 << 0
+    can_execute(program_name) = @static if is_windows()
+        ccall(:_access, Cint, (Ptr{UInt8}, Cint), program_name, X_OK) == 0
+    else
+        ccall(:access, Cint, (Ptr{UInt8}, Cint), program_name, X_OK) == 0
+    end
+
+    # If prog has a slash, we know the user wants to determine whether the given
+    # file exists and is executable, and to not search the `PATH`.  Note that
+    # Windows can have either `\\` or `/` in its paths:
+    dirseps = @static if is_windows() ["/", "\\"] else ["/"] end
+
+    if any(contains.(program_name, dirseps))
+        # If this file does not even exist, fail out
+        if !isfile(program_name)
+            throw(ArgumentError("$program_name does not exist"))
+        end
+
+        # If it does exist, check that it's executable or fail out
+        if !can_execute(program_name)
+            throw(ArgumentError("$program_name is not executable"))
+        end
+
+        # If it all checks out, return the abspath
+        return abspath(program_name)
+    end
+
+    # If we have been given just a program name (not a relative or absolute
+    # path) then we should search `PATH` for it here:
+    path = get(ENV, "PATH", "")
+    pathsep = @static if is_windows() ';' else ':' end
+
+    # On windows, we need to check both $(program_name) and $(program_name).exe
+    program_matches(file, program_name) = @static if is_windows()
+        file == program_name || file == "$(program_name).exe"
+    else
+        file == program_name
+    end
+
+    for dir in split(path, pathsep), file in readdir(dir)
+        # If we find something that matches our name and we can execute
+        if program_matches(file, program_name) && can_execute(file)
+            return abspath(file)
+        end
+    end
+
+    # If we couldn't find anything, complain
+    throw(ArgumentError("$program_name not found"))
+end
+
 end # module Sys

test/spawn.jl

@@ -529,3 +529,50 @@ end
 let s = "   \$abc   "
     @test s[Base.shell_parse(s)[2]] == "abc"
 end
+
+# Sys.which() testing
+withenv("PATH" => Sys.BINDIR) do
+    julia_exe = abspath(joinpath(Sys.BINDIR, "julia"))
+    @static if Sys.is_windows()
+        julia_exe *= ".exe"
+    end
+
+    @test Sys.which("julia") == julia_exe
+    @test Sys.which(julia_exe) == julia_exe
+end
+
+mktempdir() do dir
+    withenv("PATH" => dir) do
+        # Test that files lacking executable permissions fail Sys.which
+        foo_path = abspath(joinpath(dir, "foo"))
+        touch(foo_path)
+        chmod(foo_path, 0o777)
+        @test Sys.which("foo") == foo_path
+        @test Sys.which(foo_path) == foo_path
+
+        chmod(foo_path, 0o666)
+        @test_throws ArgumentError Sys.which("foo")
+        @test_throws ArgumentError Sys.which(foo_path)
+
+        # Test that completely missing files also fail
+        @test_throws ArgumentError Sys.which("this_is_not_a_command")
+    end
+end
+
+mktempdir() do dir
+    pathsep = @static if is_windows() ";" else ":" end
+    withenv("PATH" => "$(dir)/bin1$(pathsep)$(dir)/bin2") do
+        # Test that we have proper priorities
+        foo1_path = abspath(joinpath(dir, "bin1", "foo"))
+        foo2_path = abspath(joinpath(dir, "bin2", "foo"))
+
+        touch(foo1_path)
+        touch(foo2_path)
+        chmod(foo1_path, 0o777)
+        chmod(foo2_path, 0o777)
+
+        @test Sys.which("foo") == foo1_path
+        chmod(foo1_path, 0o666)
+        @test Sys.which("foo") == foo2_path
+    end
+end