What is "a.out"

a.out is a file format used in older versions of Unix-like computer operating systems for executables, object code, and, in later systems, shared libraries. The name stands for " assembler output", and was coined by Ken Thompson as the fixed name for output of his PDP-7 assembler in 1968. The term was subsequently applied to the format of the resulting file, to contrast with other formats for object code.

"a.out" remains the default output file name for executables created by certain compilers and linkers when no output name is specified, even though the created files actually are not in the a.out format.