Learning compound commands

krytron · 10-10-2009, 04:01 PM

Hello, this is my first time posting.

I'm learning compound commands. The article I'm reading gave some simple examples on how to use them, for instance:

(cd Documents; ls); ls

but I can accomplish the same thing by typing:

ls Documents; ls

There are other commands I've tried and all of them have an alternative to using parentheses and require less typing. So I'm wondering, if using compound commands is only useful for more advanced commands or shell scripts? I haven't tried it yet but I guess if you were to install updates in the shell you could go:

(sudo apt-get install updates)

That way it would run in the sub shell and you could continue working in the parent shell.

pixellany · 10-10-2009, 04:05 PM

Welcome to LQ!!

I'm not really clear what your question is but it think you can see how this works by just trying it.

Your two examples are NOT the same---in the first case, you will be left in "Documents"--in the second case, you'll stay in the parent directory.

krytron · 10-10-2009, 04:28 PM

Hi pixellany, thank you for the welcome

Well the commands are different but have the same outcome. After I type in both examples I remain in my home directory. In both cases my prompt doesn't change and I also checked by typing "pwd" after both examples and both times I remained in my home directory. In the first example, the first part of the command in executed in the subshell and doesn't affect the working directory.

I get the same outcome in both examples, only the latter requires less typing.
So that's what I'm wondering, what is using parentheses good for? More advanced stuff like shell scripting?

Tinkster · 10-10-2009, 04:43 PM

The differences become more obvious once you start using shell variables, or modifying them.
The first version would be executed in a sub-shell, and leave the calling shells variables
intact. The second would 'permanently' change them. The visible result in your example is
the same. The 'hidden results' are different. In your first example $PWD would have had
different values during the execution.

Cheers,
Tink

Elv13 · 10-10-2009, 05:17 PM

{ } and ( ) are used to isolate a group of commands from the parent script, they will have no impact on any variables as explained earlier. You can also pipe the output of the { } or ( ) bloc as a single command.

Ex:

Code:

{ echo apples
echo bananas
echo oranges } | grep an

will print:
bananas
oranges

you can also use it to hide some unwanted printing:

Code:

echo strawberries
{ echo apples
echo bananas
echo oranges } > /dev/null
echo grapes

will print only strawberries and grapes, evn if each command were executed.

I hope it make a little more obvious what brace and parenthesis do in bash.

krytron · 10-10-2009, 05:44 PM

Tinkster-I sort of understand what you are saying but I lack the understanding of technical terms such as shell variables.

Elv13-I understand your echo examples but I'm still confused.

Would you guys be willing to recommend some articles, tutorials, books free or not, that will give me a solid understanding of the bash shell. I've read a bit of both rute and intro to linux from tldp.org but seem to get overwhelmed with them. But if those two books are the best for beginners I will read them.

Thanks for the help!

lutusp · 10-10-2009, 06:07 PM

Quote:

Originally Posted by krytron

Tinkster-I sort of understand what you are saying but I lack the understanding of technical terms such as shell variables.

Elv13-I understand your echo examples but I'm still confused.

Would you guys be willing to recommend some articles, tutorials, books free or not, that will give me a solid understanding of the bash shell. I've read a bit of both rute and intro to linux from tldp.org but seem to get overwhelmed with them. But if those two books are the best for beginners I will read them.

Thanks for the help!

There are plenty of excellent Bash/shell tutorials. Here's mine:

Bash Shell Programming in Linux

I emphasize there are many others just as good or better.

Elv13 · 10-10-2009, 06:20 PM

This is the best guide I know of:
http://tldp.org/LDP/abs/html/index.html

mrrangerman · 10-10-2009, 06:41 PM

Here is a good start Rute

You can also use the {} braces for shell/brace expansion. Say you want to make several directories with a common name and postfix them with numbers. You can do mkdir name1 then mkdir name2 and so on or you can take advantage of shell expansion and use the command

Code:

mkdir name{1..9}

and this will create all the directories from name1 through name9 with one command. You can prefix in the same way

Code:

mkdir {1..9}name

.
This will also work in making multiple files, with the touch command

Code:

touch file{1..9}.txt

cola · 10-10-2009, 10:30 PM

http://ss64.com/bash/

krytron · 10-11-2009, 01:05 AM

Thank you everyone for your help and suggestions.