How To Create an Alias in Unix shell

When you want to save yourself from typing an unwieldy command over and over again you can create and use an alias for it. It will then act as a shortcut to the larger command, which you can type and run instead.

Creating aliases in UNIX (and Linux) is done with a simple alias command which follows this format: alias name='command you want to run'.

Replace the "name" with your shortcut command, and "command you want to run" with the larger command you want to create an alias of. Here's a simple example:

alias accesslog='tail -f /var/log/lighttpd/access.log'  

In this example I've effectively created a new accesslog command which is an alias of the tail -f /var/log/lighttpd/access.log command. What it does is follow the access.log file and display new entries in it as they happen. Now instead of having to write the whole tail -f command every time I want to look at what's happening in the access.log file I can simply run the accesslog alias command instead, which is pretty nifty.

What if I want to unset the alias once I no longer need it or wish to set a new better alias? Well, simply run:

unalias accesslog  

Quite logical. Now the accesslog alias no longer exists.

One thing to keep in mind though is that aliases that are set this way get lost the moment you close the command line session, or in other words, they are temporary. If you want to save aliases permanently you will have to edit the bash configuration file, which is usually .bashrc or .bash_profile residing in your user home directory. You can edit whichever you prefer, or whichever exists on your system.

To edit .bashrc just open it in a command line text editor such as nano, or any other you might prefer, and add the same exact alias command as in the above example at the bottom of it, or find where other aliases are already set and add yours after them.

nano .bashrc  

Once you add your aliases save the file, which in the nano editor is done by pressing the Сtrl-x keyboard shortcut, answering "y" when asked to save, and hitting enter.

Now your alias is saved permanently, and it will therefore work even after you close the session and come back. Of course, to remove the permanent alias just edit the file again and remove the line you've just added. If it's still set run the unalias command as shown above and it will be gone.

Note that aliases are set for the currently active user. So you have to edit the .bashrc file in the home directory of that user. If you're logged in as root that would be /root/.bashrc, and if you're logged in as joe, for example, it will be in /home/joe/.bashrc. If you try to run root's alias while acting as joe or vice versa you'll get a "command not found" error.

Also note that aliases added to .bashrc aren't active immediately after you save the file since that file is read on user's login. If you log out and log back in then it will work.

Finally, once you have a bunch of aliases set up you might want to check up on which aliases are available. To do that just run the alias command by itself:

alias  

And it will list something like this:

alias accesslog='tail -f /var/log/lighttpd/access.log' 
alias ls='ls --color=auto'  

The list represents all of the aliases that have been set in .bashrc, or on the command line during the current session. In the above example we see my accesslog alias, and another one for the ls command associating it with the ls –color=auto command, which simply adds some coloring to our ls lists.

That brings us to the final point worth a mention, as demonstrated by the above ls alias, and that is that you can alias an already existing real command. For example if we have a nmon command installed, which shows various system activity information, we can actually turn it into an alias for the top command, which also shows system activity.

You probably don't want to do this, or at least, you don't want to keep this alias, but for the sake of demonstration:

alias nmon='top'  

And now when you run nmon, instead of opening the actual nmon program it will open top. In other words the alias is masking the original command.

This serves as a word of caution when it comes to setting names of aliases; try to avoid setting names that match existing commands. Chances are you'll want those commands doing what they're supposed to do, except in special cases like the above ls alias, which simply aliases to its own coloring options.

And that's how aliases work in UNIX (and Linux).

Using Dropbox with Unix

Although last week saw some pretty exciting developments in the cloud storage (Google Drive announcement and SkyDrive free 25Gb space), the truth is that Dropbox is still the king of the cloud storage hill – it's hands down the easiest to use and integrate.

I've been a Dropbox user for a few years now, but have started using it actively only in the last 12 months or so. It's been an invaluable tool for me thanks to its integration with 1Password, the password tool of my choice. Dropbox also helps with lots of day-to-day tasks and thats why I decided it's time to share some of the tips.

Having used Dropbox extensively on Windows systems (XP on laptop and Win7 on desktops), I've recently moved on to using Dropbox with my Mac OSX desktop and Linux hosting.

So here are the top tips for using Dropbox with Unix – each one does wonders for me and so I hope you like them as well.

Important: If you're not a Dropbox user yet, please use this link to sign up – it means I'll get a small bonus (extra 500MB to my free account) for referring you.

[Read more…]

Mounting NFS shares on Mac OS X

I've recently decided to give Mac OS X a try. For the past week or so I've been spending a good few hours a day working in Snow Leopard installed on a MacBook Pro borrowed from a friend.

While Mac OS is unlike any Unix-like operating system I've managed so far, there are certainly some of similarities. I can honestly say that I'm enjoying the Mac Book Pro so far, and hope to discover most of the differences compared to my previous Unix-like desktop which is Ubuntu 9.10.

Mounting NFS on MAC OS X

One thing which I noticed immediately was that out of the box it was impossible to mount any NFS shares from my Ubuntu NAS server. Any attempt to mount a remote filesystem would give me an error like this:

mbp:~ root# mount nasbox:/try /mnt
mount_nfs: /mnt: Operation not permitted

[Read more…]

Unix Tutorial membership – registration open for a week

Hi everyone!

I'm back from my holidays, and it's time to open the Unix Tutorial members area for new registrations once again!

If you're still looking for a reason to become a member – please read the Becoming a Unix Tutorial member post.

This time around, new users will receive a life-time access to just one course, the Unix foundations one. This is going to be a constantly updated and expanded course so I'm sure you'll benefit from revisiting it now and then. New courses added at a later stage will never be available for free: I plan to make them very specialized and will charge for access – more on this later.

Wait no more – here's your chance to join: Unix Tutorial members sign-up. I don't have nearly as much time to further develop the course, but please take time to leave a comment to this post or update a thread on the Unix Tutorial members discussion board about what exactly you're interested in.

Fixed calculations in Unix scripts

Although I've already shown you how to sum numbers up in bash, I only covered the bash way of doing it. I really like scripting with bash, but when it comes to calculations, there's quite a few important features missing from bash, and fixed point (thanks for the correction, Azrael Tod!) calculations is one of them. Fortunately, bc command comes as a standard in most Unix distros, and can be used for quite complex calculations.

Basic calculations with bc

bc is a very simple command. It takes standard input as an expression and then evaluates this, performing all the necessary calculations and showing you the result. Thus, to quickly sum numbers up or get a result of some other calculation, simply echo the expression and then pipe it out to the bc command:

ubuntu$ echo "1+2" | bc
3

[Read more…]

Command Aliases in Unix shells

One of the really useful features almost every Unix shell has is support for command aliases – a way to run a command or a series of Unix commands using a shorter name you get associated with such commands.

An example of a command alias in Unix shell

Here's one of the most useful aliases I have for Solaris systems:

solaris$ alias ls='/usr/local/gnu/bin/ls --color -F'

[Read more…]

Converting date and time to Unix epoch in Perl

Today I was working on a script, and one of the subroutines needed simple seconds-based arithmetics with time. As you probably remember from  my date and time in Unix scripts article, the easiest way to approach this task is to deal with the raw representation of date and time in Unix – the Unix epoch times. This post will show you how to convert standard dates into Unix epoch times in Perl.

[Read more…]

How To Change Ownership of Files and Directories in Unix

I've just been asked a question about changing the ownership of files from one Unix user to another, and thought it probably makes sense to have a quick post on it.

File ownership in Unix

Just to give you a quick reminder, I'd like to confirm that every single file in Unix belongs to some user and some group. There simply isn't a way to create a file without assigning ownership. I've briefly touched the topic of confirming file ownership in Unix before, so today I will simply build on that and show you how to change ownership of files.

[Read more…]