Linux basics with Lonestar

Logging into a remote computer using SSH

Secure Shell, or "SSH" is an executable program that runs on your local computer and allows you to connect securely to a remote computer. It supports encryption, strong passwords, and a host of other technologies that keep your information safe.

To SSH into a remote computer, you need it's location and a set of login credentials.  For Lonestar, here is the information:

Usually Windows users will need to type this information into separate boxes.  Mac OS X and Linux users can just type it straight into the Terminal.  Let's try it now by entering this:

ssh <your TACC user ID>@lonestar.tacc.utexas.edu

When prompted, type in your password.  It won't show your password on the screen, but when you press Enter, it will either accept the password or ask for it again.

When you log in to a Linux computer, the operating system checks your login credentials and if they're OK it sets up some configuration for you and then runs a program called a "shell" which acts like your fast-food drive-thru window to the rest of the operating system. You type commands and hit "enter" to send something into the drive-thru window, and then the OS passes output back through the drive-thru window.

Every time you exchange stuff through this window, it's within a context, like one specific drive-thru window at one restaurant. The directory within the file system is one part of that context; the programs and environment variables available to you are other parts of that context. When you log in, the system and shell agree that you'll start off in your home directory on the system.

Essential command-line tricks to look like an expert quickly

Type as little and as accurately as possible by cheating:

• Cheat 1: Use "up arrow" to retrieve any of the last 500 commands you've typed. You can then edit them and hit enter (even in the middle of the command) and the shell will use that command.

• Cheat 2: Hit the tab key twice - it's almost always magic. This instructs the shell to try to guess what you're doing and finish the typing for you. On most modern Linux shells, it works for commands (like "ls" or "scp") and for completing file or directory names.

• Cheat 3: You can use control-a (holding down the "control" key and "a") to jump the cursor right to the beginning of the line. The omega to that alpha is control-e, which jumps the cursor to the end of the line. Arrow keys work, and control-arrowl will skip by word forward and backward.

Unfortunately, you are pretty much out of luck if you want to jump to the middle of the line. In this case you might want to copy the whole command into a nice text editor on your desktop, change it, and copy it back.

Advanced topic: command line editors.

Exercise:

Type "modu" then hit tab twice - it presents two choices, module and modutil. Type the next character l, hit tab twice and it will complete the rest of the typing. If you hit tab twice again, the OS will show you all the files in your current working directory which doesn't make any sense for the command "module" - it's smart, but not smart enough to figure out that the next word in the command needs to be one of module's built-in commands.

Inline help

Man pages - Linux has had built-in help files since the mid-1500's, way before Macs or PCs thought of such things. In Linux they're called man pages - short for "manual"; it's not a gender thing (I assume). man intro will give you an introduction to all user commands.

Exercise:

Try "man grep", or "man du", or "man sort" - you'll want these sometime.

Tip: Type the letter q to quit man, j and k/<CR> to move up and down by line, b or spacebar up/down by page. Want to search? Just hit the slash key /, enter the search word and hit enter. These are actually the tools of the less command which man is using.

Basic Linux commands you need to know like breathing air

• ls - list the contents of the current directory
• pwd - print the present working directory - which restaurant am I at right now - the format is something like /home/myID - just like on most computer systems, this represents leaves on the tree of the file system structure, also called a "path".
• cd <whereto> - change the present working directory to <whereto> - pick up my drive-thru window (shell) and move it so that I'm now looking thru to the directory <whereto>
  • Some special <wheretos>: .. (period, period) means "up one level". ~ (tilde) means "my home directory". ~myfriend (tilde "myfriend) means "myfriend's home directory".
• df shows you the top level of the directory structure of the system you're working on, along with how much disk space is available
• head <file> and tail <file> shows you the top or bottom 10 lines of a file <file>
• more <file> and less <file> both display the contents of <file> in nice ways. Read the bit above about man to figure out how to navigate and search when using less
• file <file> tells you what kind of file <file> is.
• cat <file> outputs all the contents of <file> - CAUTION - only use on small files.
• rm <file> deletes a file. This is permanent - not a "trash can" deletion.
• cp <source> <destination> copies the file source to the location and/or file name destination}. Using . (period) means "here, with the same name". * cp -r <dirname> <destination> will recursively copy the directory dirname and all its contents to the directory destination.
• scp <user>@<host>:<source> <destination> works just like cp but copies source from the user user's directory on remote machine host to the local file destination
• mkdir <dirname> and rmdir <dirname> make and remove the directory "dirname". This only removes empty directories - "rm -r <dirname>" will remove everything.
• wget <url> fetches a file with a valid URL. It's not that common but we'll use wget to pull data from one of TACC's web-based storage devices.

Wildcards and special file names.

The shell has shorthand to refer to groups of files by allowing wildcards in file names. * (asterisk) is the most common; it is a wildcard meaning "any length of any characters". Other useful ones are [] to allow for any character in the set <characters> and {} for a range of characters.

For example: ls *.bam lists all files in the current directory that end in .bam; ls [n-z]*.bam does the same, but only if the first character of the file is a letter between "n" and "z" in the alphabet.

Three special file names:

1. . (single period) means "this directory".
2. .. (two periods) means "directory above current." So ls -l .. means "list contents of the parent directory."
3. ~ (tilde) means "my home directory".

Exercises:

Scavenger hunt practice; on Lonestar issue the following commands:

cp -r /corral-repl/utexas/BioITeam/linuxpractice .
cd linuxpractice
cd what
cat readme

and follow the instructions. Hints: use <tab><tab> to fill in filenames as much as you can.

pwd
here=`pwd`
cd /scratch/01057
pwd
cd $here
pwd

Options: the lifeblood of Linux commands

Sitting at the computer, you should have some idea what you need to do. There's probably a command to do it. If you have some idea what it starts with, you can type a few characters and hit tab twice to get some help. If you have no idea, you google it or ask someone else. But soon you want those commands to do a bit more - like seeing the sizes of files in addition to their names.

Most commands in Linux use a common syntax to ask more of a command; they usually add a dash "-" followed by a code letter that means "do the basic command, but with a bit more..."

ls -l
ls -lh
ls -t

These little toggle-like things are often called "command line switches"; there can be other options, like filenames, that aren't switches.

Almost all commands, and especially NGS tools, use options heavily.

Like dialects in a language, there are at least three basic schemes commands/programs accept options in:

1. One letter options which can sometimes be combined, or other single options like:
   Examples of different option types

   head -10
   ls -lhtS (equivalent to ls -l -h -t -S)
   

2. Word options, like -d64 and -Xms512m in this command, that are never combined (this is the GATK command to call SNPs):
   Examples of word options

   java -d64 -Xms512m -Xmx4g -jar /work/01866/phr254/gshare/Tools_And_Programs/bin/GenomeAnalysisTK.jar -glm BOTH -R $reference -T UnifiedGenotyper -I
   $outprefix.realigned.recal.bam --dbsnp $dbsnp -o $outprefix.snps.vcf -metrics snps.metrics -stand_call_conf 50.0 -stand_emit_conf 10.0 -dcov 1000
   -A DepthOfCoverage -A AlleleBalance
   

3. "Long option" forms, using the convention that a single dash - precedes single-letter options, and double dashes- - precede word options, like this command to run the mira assembler:
   Example of long options

   mira --project=ct --job=denovo,genome,accurate,454 -SK:not=8
   

man pages should detail all options available for a command. Unless there's no man page.

More help please

Sometimes man lets you down - no man page. Don't fret, try one of these:

1. Just type in the command and hit return - it will usually try to help you.
2. Type the command followed by one of: -h -help --help -? and may give you some help.
   Sometimes the command by itself will give you short help, and will list the magic option for full help.

Next Section: Lonestar Essentials