Shell Programming and Scripting

oops...

I guess we are assuming echo is built-in and not /bin/echo (GNU external).

Code:

[user@host ~]$ echo --version
--version
[user@host ~]$ /bin/echo --version
echo (GNU coreutils) 6.10
Copyright (C) 2008 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

It has been built into every shell for probably about 20 years. POSIX requires that such commands also be available as external commands.

printf, which is recommended over echo, is built into all major POSIX shells.

I realize it is built-in and has been for a while. I just get so used to using complete paths when writing scripts that I overlook the built-ins from time to time.

I have seen more than a few scripts fail because the ${PATH} var is not set as expected when running in a cron or run remotely.

A quick way around that is to use full paths for all commands and when you get used to that technique in a script... built-in commands get used far less then their external counterparts.

Another thing that I notice is that the built-ins behave differently than expect in some cases since they are usually not the full replica of their external counterparts.

Take for example my previously posted echo example with the --version flag.


Either way, I agree that more efficient is better and certainly your example was shorter and has less commands than mine.

I was just trying to point out that the parameter expansion constructs are not always the easiest to remember and that their ARE alternatives. Some are far easier to remember as well. Efficient is also from the view point of the person writing the script. How efficient is it when you have to go read man pages or google and answer and you spend 20-30 extra minutes writing a single line because you cannot remember the exact method for the parameter expansion you want to use?

Absolutely. String operations can also be difficult to apply to a given problem when there isn't one to do exactly what you want. In any case, it's useful to have a list of them. Since bash 3, it supports built-in regular expressions too... I'll have to disagree with you on that one. In lower-level languages like C, the easier way is often better because its clearer and simpler and not that much slower in any case. But the cost of launching a process is enormous, you should use processes for:Launching lots of short-lived processes for tiny things can easily magnify the running time of your script to the point where it takes huge amounts of time to do anything non-trivial.

More than "a while": for the entire life of the Bourne shell and its successors.

I have seen more than a few scripts fail because the full path was used. It's far easier to adjust the PATH, if necessary, than to change all the commands when running a script on a different system.

In an efficient script, it is quite the opposite. Many of my scripts call no more than one external command, and many call none.

I haven't seen that.

The use of echo is deprecated, and it should not be used when the contents of its arguments are not known in advance.

20-30 minutes to do: "man bash / Parameter Expansion <ENTER>"???
You must be joking!!

You might want to use my sman function (see below).

Besides, after using them a few times, you will not have to look them up, and your scripts will be much faster. Learn them now before bash4 is released; it has many more parameter expansions (some of which I suggested).

Code:

sman() ##display a man page with a search 
{
    LESS="$LESS${2:+ +/$2}" man "$1"
}

Since I'm in computer science I asked my teacher, he showed me the basename method, worked like a charm.

Thanks for the
echo "${PWD##*/}"

Will try understanding what does ##*/ mean.

Please don't misunderstand me, I agree with you on the points you make. I am not disputing these points. I am merely stating that for a new user, some of the conventions you use here are daunting to say the least. Some are also quite simple and elegant but to the beginner something like "${VAR%/*}" is greek

That said, yes... your scripts and you skill level will improve the more you use better scripting habits. Never has been a question there either.

I would suggest to any that wish to learn that they take all examples you and others give and learn the differences as they get more comfortable.

The 30 minute reference was meant to refer to time spent doing google searches. A lot of beginners neglect man pages. Either from lack of knowledge of their existence or from lack of good habbit.


Hey nice function (sman) by the way.