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Remembering all sorts of commands and their options
and typing them each time is tedious work. Fortunately you don't have to.
You can define shortcuts for frequently used commands. These shortcuts can
either be defined in the relatively simple form of command aliases or in
the somewhat more complex syntax of shell functions.
Command Aliases
For example, I use this command to upload my stuff to
MUO:
rsync -e ssh -z -t -r -vv --progress /home/tom/web/muo/rsmuo/docs
muo:/www/mandrakeuser/docs
Obviously I'd go nuts if I had to type this command
line each time. So I have defined what is called an 'alias' in '~/.bashrc':
alias upmuo='rsync -e ssh -z -t -r -vv --progress
/home/tom/web/muo/rsmuo/docs muo:/www/mandrakeuser/docs'
Now all I have to do to upload my stuff is to type upmuo.
The syntax for defining an alias is:
alias shortcut='command'
The quotes are necessary when the command contains empty
spaces (e.g. between command and option). Notice that you either quote using
single quotes or using double quotes. If you've got a hitch that there's
a difference between those two, then you are right ;-).
Single quotes strip the special meaning from all
characters included, double quotes from all characters except for
'$' (parameter substitution) and '`' (command substitution). Which means
in order to use variables or command substitution in aliases, you have to
use double quotes. If you look at the example above, I could define a variable
called MUOHOME in '.bashrc':
export MUOHOME=$HOME/web/muo/rsmuo/docs
To use that variable in the alias above, I would have
to use double quotes:
alias upmuo="rsync -e ssh -z -t -r -vv --progress
$MUOHOME muo:/www/mandrakeuser/docs"
because otherwise the alias would search for an directory
or file called '$MUOHOME'.
You can create aliases 'on the fly' with the 'alias'
command on the command line, or list them in '~/.bashrc' (per user), or in
'/etc/profile.d/alias.sh' (for every user and 'root'), in pre-8 Mandrake
Linux releases, '/etc/bashrc' fulfills that function. To delete an alias,
simply type: unalias alias. Just running alias
will list all the defined aliases on your system.
If you have a look at '~/.bashrc' and '/etc/profile.d/alias.sh',
you'll see that there are already some aliases defined. You can define more
than one alias for the same command. Of course, you must make sure that your
alias isn't the name of some other program, something like alias rm='ls
-l' won't work. You can try this by typing the shortcut you want to
use on the command line. If the shell can't find a command with this name,
you can use it as an alias.
Some aliases that might be useful (don't forget the
quotes!):
alias rpmq='rpm -qa | grep'. Now rpmq
string will list all installed RPMs
which contain string in their name.alias ls='ls -ho --color | more'. ls
will now print a colored and paged listing with file sizes in KB.alias use='du --max-depth=1 | sort -n | more'.
use gives you a paged list of subdirectory
sizes ordered by size.alias dkd='cd /usr/src/linux/Documentation'.
Frequently used directories can be aliased as well. Other prospective candidates
are for example the subdirectories of '/mnt'.
Aliased directories may also be on removable media: alias dlm='/mnt/cdrom/Mandrake/RPMS/'.
One mnemonic hint: try to start aliases with a similar
function always with the same letter. For instance, begin all your directory
aliases with a 'd'.
I am sure you will find lots of possibilities for using
this feature.
Shell Functions
Writing shell functions already boarders on the topic
of shell scripting, which is beyond the scope of this article (and mine ;-)).
In fact, shell functions are shell scripts, but they have the advantage
of being preloaded and being executed in the same shell (whereas a shell
script opens at least one sub-shell).
With shell functions, you can do a lot of things you
can't do with aliases. One example:
function apros() { apropos $1 | egrep -v '(3|\(n\)'; }
This defines a new command, called 'apros'. apros
name will execute 'apropos name'
(i.e. a search command for man pages) and pipe (|) the output of that command
through 'egrep' which filters out all man pages from sections '3' and 'n',
which usually are not of interest, but tend to mess up the output of the
'apropos' command.
Functions allow you to use arguments given to the function name at any place
of the function command. With aliases, only one argument is allowed and that
argument has to be at the end of the command line (like in the 'rpmq' alias
above).
'$1' is a so-called 'positional parameter', it's a placeholder for the first
argument given to the function. Of course there are more.
function apros() { apropos $1 | egrep -v "\($2";
}
If you now run the 'apros' command like this:
apros name man_section_number
it searches for name, but excludes all man
pages from man_section_number:
apros menu 3
returns all man page titles which contain 'menu', except
those from section 3 (programming). Notice that you have to quote twice
and that you have to use double quotes:
- You must quote the 'egrep' search pattern in order
to protect it from the shell.
- You must use double quotes to get the second positional
parameter interpreted correctly.
- You must quote the round bracket again in order to
tell 'egrep' to take it literally and not as a special control character
.
Tricky, ain't it? ;-).
Shell functions are handled just like aliases: put them
into your '.bashrc' to have them around permanently.
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This article is but the beginning. Shell scripting can
help you to automate a lot of tasks, to fix errors in scripts by others yourself
and to accustom your Mandrake Linux system to your liking in (almost) every
aspect. If you plan to learn one of the more complex programming languages
out there, shell scripting is a good place to start because the basic concepts
are similar.
The BASH
Programming - Introduction HOW-TO will explain the topics covered in
this article in more depth and introduce you to the world of shell programming.
You can then continue with the very recommendable (and free) Advanced Bash-Scripting
Guide by Mendel Cooper.
If you prefer books, I can recommend S. Veeraraghavan's
'Teach Yourself Shell Programming', Sams Publishing. In contrast, I found
'Learning the bash Shell' by Newham/Rosenblatt, O'Reilly, rather unhelpful
and confusing, but maybe that's just me ;-).
Apart from that: practice, practice, practice. Read
shell scripts by others and try to understand what they do, how and why.
Don't run your test scripts as 'root'. Have fun.
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How do I turn that &*#! beep off?
With this command:
setterm -blength 0
Why do I get bash: command: command
not found?
If it isn't a typo, most likely the command you are
trying to execute is located in a directory which is not part of your $PATH.
Supply the full path to the command. If you are in the same directory as
the command, do ./command.
Why do I get bash: command: Permission
denied?
In order for a file to be executable, the execute permission must be set for the user
who wants to execute the file. Do a:
chmod 755 file
If that doesn't work, read the article on permissions.
The execution bit is set, I have permissions to execute
but permission is denied anyway. Why?
Check the '/etc/fstab' entry of the partition where that
file is. Make sure it doesn't contain the option 'noexec'. If it contains
the option 'user', the option 'exec' must be set, too.
How do I change the file listing colors?
Copy '/etc/DIR_COLORS' to your home directory and rename
it to '.dir_colors'. Everything you need you'll find in that file.
I've put a script 'foo' into my '~/bin' directory,
but every time I try to run it, a different command also called 'foo' is
started. Why?
Have a look at your $PATH and you will see that your
personal '~/bin' directory is the last or at least very close to the end
of the $PATH. The shell will search the directories listed in $PATH one after
the other. As soon as it finds the first matching command, it executes this
command. If there is a command on your system with the same name as your
script, it is likely that this command will be executed, not your script.
So rename your script.
What does bash: command: bad interpreter
mean?
This usually happens with third party installation binaries
or Java applications. These applications need a shell of their own, so instead
of
command
you have to run
shcommand
My terminal freezes everytime I press <CTRL
s>!
Don't do that then ;-). <CTRL s> sends
a scroll lock command to the terminal. To release this lock, just press <CTRL
q>.
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