Programming Praxis

I have mentioned previously that I work for a community college, part of a team maintaining an enterprise-wide database system on Oracle and Unix. My current assignment involves a file-transfer program between us and the Department of Education (they don’t use sftp like the rest of the world), and I am writing in the Posix shell language because that is the only language that can both be called from our user interface and run the program that the Department of Education requires. It’s not a big program, under five hundred lines, but there’s lots going on. Here are some examples:

• Text file database: The Department of Education sends us a file of fixed-length records terminated by carriage-returns and newlines containing ascii text that acts as a database for holding configuration metadata. I read the database by selecting the desired record with grep and extracting the needed field with cut -c.
• Strip file headers/trailers: Files received from the Department of Education have header and trailer lines added to the data. I strip those lines with an ed script:
  

  ed $FILENAME <
  

• Arithmetic: At one point during the development of the program I needed to do some arithmetic, nothing fancy. That requirement has now gone away, but at the time I used bc to do the arithmetic, passing input using shell variables and returning output to a shell variable. And I couldn’t resist; the solutions page has a factoring program written in bc.
• Oracle database: I use SQL*Plus to insert and query records in the Oracle database.
• Shell built-ins: I use many of the built-in shell commands. If and test allow me to execute commands conditionally. While and do let me do loops. Cp and mv let me get things where they belong. Chown and chmod let me control the security of my data. Read lets me index through a file line-by-line. Shell variables let me parameterize the code. Shell functions let me modularize the code.

I’m not the first person to remark that having a single unifying datatype — ascii text in delimited files — and an assortment of programs to operate on that datatype makes a wonderfully useful system environment.

Your task is to tell us about your use of unix command-line utilities and shell scripts; hopefully other readers will be inspired by something interesting that you have done. When you are finished, you are welcome to read a suggested solution, or to discuss the exercise in the comments below.

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