Original author(s)  Brian Kernighan, Lorinda Cherry (AT&T Bell Laboratories) 

Developer(s)  Various opensource and commercial developers 
Initial release  June 1974 
Written in  C, Yacc^{[1]} 
Operating system  Unix, Unixlike, Plan 9 
Platform  Crossplatform 
Type  Command 
License  Plan 9: MIT License 
Part of the troff suite of Unix document layout tools, eqn is a preprocessor that formats equations for printing. A similar program, neqn, accepted the same input as eqn, but produced output tuned to look better in nroff. The eqn program was created in 1974 by Brian Kernighan and Lorinda Cherry. It was implemented using yacc compilercompiler.^{[1]}
The input language used by eqn allows the user to write mathematical expressions in much the same way as they would be spoken aloud. The language is defined by a contextfree grammar, together with operator precedence and operator associativity rules.^{[2]} The eqn language is similar to the mathematical component of TeX, which appeared several years later, but is simpler and less complete.
An independent compatible implementation of the eqn preprocessor has been developed by GNU as part of groff, the GNU version of troff. The GNU implementation extends the original language by adding a number of new keywords such as smallover and accent. mandoc, a specialised compiler for UNIX man pages, also contains a standalone eqn parser/formatter.
Eqn was written using the yacc parser generator.^{[1]}
Here is how some examples would be written in eqn^{[3]} (with equivalents in TeX for comparison):
TeX  eqn  formula 

a^2

a sup 2


\sum_{k = 1}^N k^2

sum from { k = 1 } to N { k sup 2 }


x = {b \pm \sqrt{b^2  4ac} \over 2a}

x = {b + sqrt{b sup 2  4ac)) over 2a

Spaces are important in eqn; tokens are delimited only by whitespace characters, tildes ~, braces {} and doublequotes "". Thus f(pi r sup 2)
results in , whereas f( pi r sup 2 )
is needed to give the intended .
File system  

Processes  
User environment  
Text processing  
Shell builtins  
Networking  
Searching  
Software development  
Miscellaneous  