ARCH(1) General Commands Manual ARCH(1)

arch — print architecture type or run selected architecture of a universal binary

arch

arch [-32] [-64] [[-arch_name | -arch arch_name]...] [-c] [-d envname]... [-e envname=value]... [-h] prog [args ...]

The arch command with no arguments, displays the machine's architecture type.

The other use of the arch command is to run a selected architecture of a universal binary. A universal binary contains code that can run on different architectures. By default, the operating system will select the architecture that most closely matches the processor type. A 64-bit architecture is preferred over a 32-bit architecture on a 64-bit processor, while only 32-bit architectures can run on a 32-bit processor.

When the most natural architecture is unavailable, the operating system will try to pick another architecture. On 64-bit processors, a 32-bit architecture is tried. Otherwise, no architecture is run, and an error results.

The arch command can be used to alter the operating system's normal selection order. The most common use is to select the 32-bit architecture on a 64-bit processor, even if a 64-bit architecture is available.

The arch_name argument must be one of the currently supported architectures:

i386
32-bit intel
x86_64
64-bit intel
x86_64h
64-bit intel (haswell)
arm64
64-bit arm
arm64e
64-bit arm (Apple Silicon)

If the binary does not contain code for arch_name, the arch command may try to select a close match. If arm64 is specified and not found, arm64e will be tried next. If this happens, the order the architectures will be tried is not guaranteed.

Either prefix the architecture with a hyphen, or (for compatibility with other commands), use -arch followed by the architecture.

If more than one architecture is specified, the operating system will try each one in order, skipping an architecture that is not supported on the current processor, or is unavailable in the universal binary.

The other options are:

Add the native 32-bit architecture to the list of architectures.
Add the native 64-bit architecture to the list of architectures.
Clears the environment that will be passed to the command to be run.
envname
Deletes the named environment variable from the environment that will be passed to the command to be run.
envname=value
Assigns the given value to the named environment variable in the environment that will be passed to the command to be run. Any existing environment variable with the same name will be replaced.
Prints a usage message and exits.

The prog argument is the command to run, followed by any arguments to pass to the command. It can be a full or partial path, while a lone name will be looked up in the user's command search path.

If no architectures are specified on the command line, the arch command takes the basename of the prog argument and searches for the first property list file with that basename and the .plist suffix, in the archSettings sub-directory in each of the standard domains, in the following order:

~/Library/archSettings
User settings
/Library/archSettings
Local settings
/Network/Library/archSettings
Network settings
/System/Library/archSettings
System settings

This property list contains the architecture order preferences, as well as the full path to the real executable. Please refer to the EXAMPLES section for an example of the property list file format.

When a link is made to arch command with a different name, that name is used to find the corresponding property list file. Thus, other commands can be wrapped so that they have custom architecture selection order.

Because of some internal logic in the code, hard links to the arch command may not work quite right. It is best to avoid using hard links, and only use symbolic links to the arch command.

The environment variable ARCHPREFERENCE can be used to provide architecture order preferences. It is checked before looking for the corresponding property list file.

The value of the environment variable ARCHPREFERENCE is composed of one or more specifiers, separated by semicolons. A specifier is made up of one, two or three fields, separated by colons. Architectures specified in order, are separated by commas and make up the last (mandatory) field. The first field, if specified, is a name of a program, which selects this specifier if that name matches the program name in question. If the name field is empty or there is no name field, the specifier matches any program name. Thus, ordering of specifiers is important, and the one with no name should be last.

When the arch command is called directly, the prog name provides the path information to the executable (possibly via the command search path). When a name is specified in a ARCHPREFERENCE specifier, the path information can alternately be specified as a second field following the name. When the arch command is called indirectly via a link, this path information must be specified. If not specified as a second field in a specifier, the executable path will be looked up in the corresponding property list file.

This is an example of a property list file as is expected in one of the archSettings locations mentioned above:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
        <key>ExecutablePath</key>
        <string>$execpath</string>
        <key>PreferredOrder</key>
        <array>
                <string>x86_64</string>
                <string>arm64</string>
        </array>
        <key>PropertyListVersion</key>
        <string>1.0</string>
</dict>
</plist>

i386,x86_64,x86_64h,arm64,arm64e
A specifier that matches any name.
foo:i386,x86_64,x86_64h,arm64,arm64e
A specifier that matches the program named foo (the full executable path is in the foo.plist file).
foo:/op/bin/boo:i386,x86_64,x86_64h,arm64,arm64e
A specifier with all fields specified.
baz:i386;x86_64,x86_64h,arm64,arm64e
A specifier for baz and a second specifier that would match any other name.

machine(1)

Running the arch command on an interpreter script may not work if the interpreter is a link to the arch command.

February 15, 2021 Mac OS X 12