a.out (assembler output) is a file format, used in older versions of Unix and Unix-like operating systems, for executables, object code, and, in later systems, shared libraries. It appeared in V6 UNIX and it was superseded by the COFF format in AT&T Unix System V, which was in turn superseded by ELF in System V Release 4.

Though Berkeley Unix kept using a.out for some time, modern BSD-systems have since switched to ELF. NetBSD/i386 switched formally from a.out to ELF in its 1.5 release.

Linux also used a.out until kernel 2.0, when it was superseded by ELF for that platform as well.

a.out is still the default output name for executables created by the compiler or linker when no name is specified with the -o option.

Executable file formats

A.out | A.out | A.out