Question

Why should I learn Shell Programming at all? What can be done with it in the real world? Could you show me some powerful things that can be done with it or some special features so as to convince me that I should start learning shell programming right now?

Answer 1

There are a billion and one reasons to learn shell programming. One of the major reasons is systems administration.

Here is an example of somebody who needed to rename ~750 files based upon another file in that same directory. This was accomplished with 3 lines of shell scripting. To get many more examples just search for questions with the tags [bash], [sed] or [awk].

When somebody asks me to show them a cool example of “shell programming” I always show them this awk 1-liner (maybe even a 1-worder?). It will filter a list to only show unique values without changing the original order. This is significant because most other solutions require you to sort the list first which destroys the original order.

$ echo -e "apple\npear\napple\nbanana\nmango\npear\nbanana" | awk '!a[$0]++'
apple
pear
banana
mango

Explanation of awk command

The non-sorting unique magic happens with !a[$0]++. Since awk supports associative arrays, it uses the current record (aka line) $0 as the key to the array a[]. If that key has not been seen before, a[$0] evaluates to 0 (zero) which is awk’s default value for unset indices. We then negate this value to return TRUE on the first occurrence of that key. a[$0] is then incremented such that subsequent hits on this key will return FALSE and thus repeat values are never printed. We also exploit the fact that awk will default to print $0 (print the current record/line) if an expression returns TRUE and no further { commands } are given.

If you still don’t understand, don’t worry, this is a very terse and optimized version of what could be a much longer awk script.