This document discusses when to use shell scripts versus programming languages for writing programs and scripts. It defines shell scripts and programming languages. While shell scripts are quick to write and use familiar commands, the document outlines several issues with shell scripts such as confusing syntax, being untyped, slow performance, lack of portability, and no exception handling. It recommends using shell scripts for short command line tasks and programming languages for anything more complicated, fast, portable, or that needs testing.

1. Real Programmers Use
Programming Languages
(Not Shell Scripts)
Daniel Laeng - May 2014
daniel@laeng.org

2. About Me
I’m a programmer (not a sysadmin)
● I like simple, readable, well tested code
● My background: Ruby, Java, C, R, PHP, bash
● I try to use the right tool for the right job
● I have seen bad code in every language

3. (My) Definitions
Shell Script
● Primarily used as command line interpreter
● Can also be used to write scripts/programs
● e.g. sh, csh, bash, tcsh, ksh, powershell
● Shell language of choice: bash
Programming Language
● Primarily for writing programs
● Rich set of libraries
● Compiled or uncompiled
● e.g. C, C++, Java, Python, Ruby, Perl, PHP

4. Why Shell Scripts are Good
● Quick to write
● Use commands you know (cp, mv, ps, etc)
● Complex tasks can be done in a short script
● Easy to try out on command prompt
● Power of unix pipes for free

5. Shell Programming Issues
What’s bad about programming in shell scripts
● Confusing syntax
● Untyped (everything is a string)
● Slow
● Not always portable
● No exception handling

6. Confusing Syntax 1
Bash
● Outputs: “No”
● Comparison operator “>” does string comparison
● Should have used “-eq” instead

7. Confusing Syntax 2
● Outputs: “No”
● Is actually a syntax error!!
● $A must be wrapped in double quotes

8. Confusing Syntax!
● Non standard comparison operators
● Variables need to be quoted... sometimes
● Variables need to be prepended with "$"...
sometimes
● Syntax errors can be caused by variable
contents

9. Untyped
● All variables behave like strings
● Some integer operations are allowed
● No built in floating point operators!!
● Can’t define your own types

10. Speed - Which is faster?
18 seconds (touch is an external command)
1 second (echo is a bash builtin)

11. Slow
● Built in commands are very limited
● You must use external commands often
● External commands are SLOW
● Slow code is not obvious. This takes 6 seconds:
● Equivalent code takes 0.1 seconds in ruby

12. Not Reliably Portable
Shell on my computer != Shell your computer
● ksh, tcsh and zsh are not common
● bash is common (but not universal)
● bourne shell (sh) is universal but weak
● None of them are likely to work on Windows
● Strong reliance on OS commands - which differ
across computers

13. No Exception Handling
● There is no built in exception handling
● There are (complicated) ways to work around
this
● Errors are hard to debug

14. When To Use Shell Scripts
Shell (Scripts)
● Working on the command line
● Wrapper to start a program
● One off hack to automate tedious task
● Keep it SHORT! (under 50 lines)
Programming Languages
● Anything with complicated logic
● Anything that needs to be fast
● Anything that needs to be portable
● Anything that should be tested

15. Generic
● Python
● Ruby
● Perl
What To Use Instead
Building / Compiling
● Make
● Rake
● Ant
● Maven
Deployment
● Capistrano
● Bamboo
● Jenkins
● Chef / Puppet
Dev-Ops
● Chef
● Puppet
● Vagrant

16. Conclusions
Understand your tools
● Shell scripts are very useful
● But - they are not a replacement for
programming languages
● Think about your choice - pick the right tool
● Happy scripting