Is Bash scripting still relevant today, other than working with and understanding legacy code? Why wouldn't I use something like Python or even Perl instead?
Python or perl scripts are fine if you are doing more complex and "algorithmic" stuff, but for automating system administration tasks, I often find that shell scripts are simpler and more concise.
But shell scripting isn't just "scripting" (as in ".sh" files with commands). A proficient unix user/sysadmin will often build one-shot commands using features other people thing are only useful in batch scripts. Actually this is something I find baffles both the Windows-types and the people that solve every problem with "perl -e".
You're right about people needing to understand bash and Unix commands - I've seen two 20+ line Perl scripts that run the equivalent of 'date -I' and 'nc' respectively.
That said, I believe bash is somewhat limited for system administration - which I know is very odd amongst system administrators - but please let me explain why.
Using the shell on a Linux based OS in 2009, you miss out on:
* filesystem events (inotify)
* hardware events (dbus)
* config for programs with tree structured configuration (lxml)
* config for programs with sqlite based configuration
* RPM / yum beyond what their command line apps expose (and perhaps dpkg/apt too, but I need to investigate this further).
...as the commands / shell function libraries to handle these are often immature compared to the equivalent APIs. As a result, you get sysadmins causing unnecessary load by polling a file repeatedly at intervals (eg, via a cron job) rather than letting the kernel tell them when their file has changed.
I'm hopefully giving a talk about using richer languages for system administration at PyCon 2010 - I'd love to hear any thoughts or opinions on these matters fron HN readers.
Apart from the reasons CrLf was pointing out, here is another one: The shell (and a set of utilities) is standardized - it's part of the POSIX standard - this is important for some organizations. If you program in the subset of bash that conforms to the standard (enforcable with bash --posix) you are programming against an interface that will likely be more stable over the coming years than Python or Perl (think Python 2->3, or the long awaited Perl 6).
As always, it's a tradeoff, i.e. the best solution depends on the problem and requirements. Many problems shouldn't be tackled with the shell, for sure...
In my experience, the main advantage of bash is conciseness for certain tasks. I don't have much experience with Python, but I have found the shell (and associated command) less verbose than Perl in certain cases.
Obvious things like "find" and "grep", but also things like doing things with the output of a command. In Perl, you need to open the command as a file handle, which takes a couple lines of code. In bash, you just use "|".
I wouldn't write a multi-thousand program in bash, though. Not again, anyway...
Good question. I can think of two tasks for which I routinely use the shell, and wouldn't really prefer one of the newer scripting languages:
1. Customizing my interactive shell, although I personally favor zsh over bash. I write shell functions and automate PATH setting and discovery where possible, so knowing how to script the startup file helps.
2. Startup scripts for software I distribute. Perl or Python would work, but then I would have to be careful about which version of the language to use. With /bin/sh, as long as I use purely POSIX constructs (which means no arrays, unfortunately), scripts work anywhere.
