the first unix operating system was written in which language
Unix In 1972, Unix was rewritten in the C programming language.[18] The migration from assembly to the higher-level language C resulted in much more portable software,[19] requiring only a relatively small amount of machine-dependent code to be replaced when porting Unix to other computing platforms. Bell Labs produced several versions of Unix that are collectively referred to as Research Unix. In 1975, the first source license for UNIX was sold to Donald B. Gillies at the University of Illinois Department of Computer Science.[20] UIUC graduate student Greg Chesson (who had worked on the UNIX kernel at Bell Labs) was instrumental in negotiating the terms of this license.[21]