Book Excerpt: Linux Programming by Example, Part 2

Concluding our tour of the V7 ls command.

The V7 ls is a relatively small program, yet it touches on many of the fundamental aspects of Unix programming: file I/O, file metadata, directory contents, users and groups, time and date values, sorting and dynamic memory management.

The most notable external difference between V7 ls and modern ls is the treatment of the -a and -l options. The V7 version has many fewer options than do modern versions; a noticeable lack is the -R recursive option.

The management of flist is a clean way to use the limited memory of the PDP-11 architecture yet still provide as much information as possible. The struct lbuf nicely abstracts the information of interest from the struct stat; this simplifies the code considerably. The code for printing the nine permission bits is compact and elegant.

Some parts of ls use surprisingly small limits, such as the upper bound of 1024 on the number of files or the buffer size of 100 in makename().

Arnold Robbins is a professional programmer and technical author. He is the long-time maintainer of gawk, the GNU Project's version of the Awk programming language. He is the author of numerous well-known technical books, such as Linux Programming by Example: The Fundamentals, recently published by Prentice Hall, as well as Unix In A Nutshell, Learning the vi Editor and Effective awk Programming, published by O'Reilly Media Inc. He is happily married with four wonderful children. Arnold is an amateur Talmudist and particularly enjoys studying the Jerusalem Talmud. He can be reached at