Unix Basics

Accessing a UNIX machine

Mac or LINUX

To log-in into the remote Linux shell, open terminal and type:

ssh -X <your_username>@<host_name>

host name is the remote server’s domain name (e.g. bbcsrv3.biotech.uconn.edu)
You will be asked to enter the password, simply type it and press enter.

To copy files To the server run the following on your workstation or laptop:

scp -r <path_to_directory> <your_username>@<host_name>:

To copy files From the server run the following on your workstation or laptop:

scp -r <your_username>@<host_name>:<path_to_directory> .


  1. Open Putty and select ssh. Download PuTTY if you do not have it.
  2. Provide the host name (the remote server’s domain name) and session name
  3. Enter your identity information
  4. username: your username
    password: your password  Nothing will show-up, simply type the password and press enter.
  5. Setup for graphics emulation. Download and install Xming if you do not have it.
  6. Use WinSCP or FileZilla for file exchange. Download and install WinSCP or FileZilla if you do not have it.


Command-Line Syntax for this Manual

Remember the UNIX/LINUX command line is case sensitive!
The hash (pound) sign “#” indicates end of a command and the start of a comment.
The notation <...> refers to variables and file names that need to be specified by the user. The symbols < and > need to be excluded.


Viewing and changing the present working directory:

pwd                # Get full path of the present working directory (same as "echo $HOME")
 ls                # Content of pwd
 ls -l             # Similar as ls, but provides additional info on files and directories
 ls -a             # Includes hidden files (.name) as well
 ls -R             # Lists subdirectories recursively
 ls -t             # Lists files in chronological ordercd <dir_name>    
 cd                # Brings you to the highest level of your home directory.
 cd ..             # Moves one directory up
 cd ../../         # Moves two directories up (and so on)
 cd -              # Go back to you were previously (before the last directory change)

The tilde symbol (~) gets interpreted as the path to your home directory. This will happen anywhere on the command line:

echo ~            # View the full (complete) path of your home
find ~            # List all your files (including everything in sub-directories)
ls ~             # List the top level files of your home directory
du -sch ~/*       # Calculate the file sizes in your home

Viewing file info, user, and host:

stat <file-name>  # Last modification time stamps, permissions, and size of a filewhoami
hostname          # Shows on which machine you are (same as "echo $HOSTNAME")

Files and directories

mkdir <dir_name>   # Creates specified directory
rmdir <dir_name>   # Removes empty directory
rm <file_name>     # Removes file name
rm -r <dir_name>   # Removes directory including its content, but asks for confirmation,'f' argument turns confirmation off
cp <name> <path>   # Copy file/directory as specified in path (-r to include content in directories)
mv <name1> <name2> # Renames directories or files
mv <name> <path>   # Moves file/directory as specified in path

Copy and paste

The methods differ depending where you are.

  • In a command line environment:

Cut last word with keyboard only

  • Ctrl+w #Press multiple times to cut more than one word
    Paste with keyboard only

In a non-command line desktop environment:




Command line <-> desktop exchange:

Copy text out of the command line and into the desktop:

Shift+Ctrl+c        or        Apple+c

Paste text from the desktop into the command line:


Shift+Ctrl+v        or        Apple+v

Handy shortcuts

Anywhere in Command Line:

up(down)_key  – scrolls through command history
history   # shows all commands you have used recently

Auto Completion:

<something-incomplete> TAB   – completes program_path/file_name

Taking control over the cursor (the pointer on the command line):

 Ctrl+a    # cursor to beginning of command line
 Ctrl+e    # cursor to end of command line
 Ctrl-w    # Cut last word 
 Ctrl+k    # cut to the end of the line
 Ctrl+y    # paste content that was cut earlier (by Ctrl-w or Ctrl-k)

When specifying file names:

“.” (dot)            – refers to the present working directory
“~” (Tilda) or “~/”  – refers to user’s home directory

Unix Help

man <something> # general help (press the 'q' key to exit) 
man wc          # manual on program 'word count' wc
wc --help       # short help on wc
soap -h         # for less standard programs

Universally available Linux commands, with detailed examples and explanations: http://www.linuxconfig.org/linux-commands

Finding Things

Finding files, directories and applications

find -name "*pattern*"            # searches for *pattern* in and below current directory
find /usr/local -name "*blast*"   # finds file names *blast* in specfied directory
find /usr/local -iname "*blast*"  # same as above, but case insensitive

Additional useful arguments: -user <user name>, -group <group name>, -ctime <number of days ago changed>

find ~ -type f -mtime -2   # finds all files you have modified in the last two days
locate <pattern>           # finds files and dirs that are written into update file
which <application_name>   # location of application
whereis <application_name> # searches for executeables in set of directories
dpkg -l | grep mypattern   # find Debian packages and refine search with grep pattern

Finding things in files

grep pattern file           # provides lines in 'file' where pattern 'appears',
# if pattern is shell function use single-quotes: 
'>'grep -H pattern          # -H prints out file name in front of pattern
grep 'pattern' file | wc    # pipes lines with pattern into word count wc
 # wc arguments: -c: show only bytes, -w: show only words,
 # -l: show only lines; help on regular expressions:
man 7 regex or man perlrefind /home/my_dir -name '*.txt' | xargs grep -c ^.*  # counts line numbers on many
 # files and records each count along with individual file
 # name; find and xargs are used to circumvent the Linux
 # wildcard limit to apply this function on thousands of files.

Permissions and Ownership

List directories and files

ls -al   # shows something like this for each file/dir: drwxrwxrwx
          # d: directory
          # rwx: read write execute
          # first triplet: user permissions (u)
          # second triplet: group permissions (g)
          # third triplet: world permissions (o)
Assign write and execute permissions to user and group
chmod ug+rx my_file
To remove all permissions from all three user groups
chmod ugo-rwx my_file
# '+' causes the permissions selected to be added
# '-' causes them to be removed
# '=' causes them to be the only permissions that the file has.
 chmod +rx public_html/ or $ chmod 755 public_html/Example for number system:

Change ownership

chown <user> <file or dir>         # changes user ownership
chgrp <group> <file or dir>        # changes group ownership
chown <user>:<group> <file or dir> # changes user & group ownership

Useful Unix Commands

df             # disk space
free -g        # memory info in Megabytes
uname -a       # shows tech info about machine
bc             # command-line calculator (to exit type 'quit')
wget <ftp>     # file download from web
/sbin/ifconfig # give IP and other network info
ln -s original_file new_file # creates symbolic link to file or directory
du -sh         # displays disk space usage of current directory
du -sh *       # displays disk space usage of individual files/directories
du -s * | sort -nr # shows disk space used by different directories/files sorted by size

Process Management


top               # view top consumers of memory and CPU (press 1 to see per-CPU statistics)
who               # Shows who is logged into system
w                 # Shows which users are logged into system and what they are doing
ps                # Shows processes running by user
ps -e             # Shows all processes on system; try also '-a' and '-x' arguments
ps aux | grep <user_name> # Shows all processes of one user
ps ax --tree      # Shows the child-parent hierarchy of all processes
ps -o %t -p <pid> # Shows how long a particular process was running. # (E.g. 6-04:30:50 means 6 days 4 hours ...)
Ctrl z <enter>    # Suspend (put to sleep) a process
fg                # Resume (wake up) a suspended process and brings it into foreground
bg                # Resume (wake up) a suspended process but keeps it running in the background.
Ctrl c            # Kills the process that is currently running in the foreground
kill <process-ID> # Kills a specific process
kill -9 <process-ID> # NOTICE: "kill -9" is a very violent approach. It does not give the process any time to perform cleanup procedures.
kill -l           # List all of the signals that can be sent to a proccess
kill -s SIGSTOP <process-ID> # Suspend (put to sleep) a specific process
kill -s SIGCONT <process-ID> # Resume (wake up) a specific process
renice -n <priority_value> # Changes the priority value, which range from 1-19,the higher the value the lower the priority, default is 10.

Text Viewing

more <my_file>  # views text, use space bar to browse, hit 'q' to exit
less <my_file>  # a more versatile text viewer than 'more', 'q' exits, 'G' end of text, 'g' beginning, '/' find forward, '?' find backwards
cat  <my_file>  # concatenates files and prints content to standard output

Text Editors

Vi and Vim

Non-graphical (terminal-based) editor. Vi is guaranteed to be available on any system. Vim is the improved version of vi.


Non-graphical or window-based editor. You still need to know keystroke commands to use it. Installed on all Linux distributions and on most other Unix systems.


A simple terminal-based editor which is default on modern Debian systems.

The Unix Shell

When you log into UNIX/LINUX system, then is starts a program called the Shell. It provides you with a working environment and interface to the operating system. Usually there are several different shell programs installed. The shell program bash is one of the most common ones.

finger <user_name> # shows which shell you are using
chsh -l # gives list of shell programs available on your system (does not work on all UNIX variants)
<shell_name> # switches to different shell

STDIN, STDOUT, STDERR, Redirections, and Wildcards

By default, UNIX commands read from standard input (STDIN) and send their output to standard out (STDOUT).
You can redirect them by using the following commands:

<beginning-of-filename>*         # * is wildcard to specify many files
ls > file                        # prints ls output into specified file
command < my_file                # uses file after '<' as STDIN
command >> my_file               # appends output of one command to file
command | tee my_file            # writes STDOUT to file and prints it to screen
command > my_file; cat my_file   # writes STDOUT to file and prints it to screen
command > /dev/null              # turns off progress info of applications by redirecting their output to /dev/null
grep my_pattern my_file | wc     # Pipes (|) output of 'grep' into 'wc'
grep my_pattern my_non_existing_file 2 > my_stderr # prints STDERR to file

Useful shell commands

cat <file1> <file2> > <cat.out>      # concatenate files in output file 'cat.out'
paste <file1> <file2> > <paste.out>  # merges lines of files and separates them by tabs (useful for tables)
cmp <file1> <file2>                  # tells you whether two files are identical
diff <fileA> <fileB>                 # finds differences between two files
head -<number> <file>                # prints first lines of a file
tail -<number> <file>                # prints last lines of a file
split -l <number> <file>             # splits lines of file into many smaller ones
csplit -f out fasta_batch "%^>%" "/^>/" "{*}" # splits fasta batch file into many files at '>'
sort <file>                          # sorts single file, many files and can merge (-m) them, -b ignores leading white space
sort -k 2,2 -k 3,3n input_file > output_file # sorts in table col 2 alphabetically and col 3 numerically, '-k' for column, '-n' for numeric
sort input_file | uniq > output_file # uniq command removes duplicates and creates file/table with unique lines/fields
join -1 1 -2 1 <table1> <table2>     # joins two tables based on specified column numbers
 # (-1 file1, 1: col1; -2: file2, col2). It assumes that join fields are sorted. If that is not the case, use the next command:
sort table1 > table1a; sort table2 > table2a; join -a 1 -t "`echo -e '\t'`" table1a table2a > table3  # '-a <table>' prints all lines of specified table!
 # Default prints only all lines the two tables have in
 # common. '-t "`echo -e '\t'`" ->' forces join to
 # use tabs as field separator in its output. Default is
 # space(s)!!!
cat my_table | cut -d , -f1-3        # cut command prints only specified sections of a table,
 # -d specifies here comma as column separator (tab is
 # default), -f specifies column numbers.


Starting a New Screen Session

screen                 # Start a new session
screen -S <some-name>  # Start a new session and gives it a name
Commands to Control Screen
Ctrl-a d #  Detach from the screen session
Ctrl-a c # Create a new window inside the screen session
Ctrl-a Space # Switch to the next window
Ctrl-a a # Switch to the window that you were previously on
Ctrl-a " # List all open windows. Double-quotes " are typed with the Shift key
Ctrl-d or type exit # Exit out of the current window. Exiting form the last window will end the screen session
Ctrl-a [ # Enters the scrolling mode. Use Page Up and Page Down keys to scroll through the window. Hit the Enter key twice to return to normal mode.

Attaching to Screen Sessions

From any computer, you can attach to a screen session after SSH-ing into a server.

screen -r              # Attaches to an existing session, if there is only one
screen -r              # Lists available sessions and their names, if there are more then one session running
screen -r <some-name>  # Attaches to a specific session
screen -r <first-few-letters-of-name> # Type just the first few letters of the name & you will attach to the session you need

Destroying Screen Sessions

1. Terminate all programs that are running in the screen session. The standard way to do that is:


2. Exit out of your shell.

3. Repeat steps 1 and 2 until you see the message:
[screen is terminating]
There may be programs running in different windows of the same screen session. That’s why you may need to terminate programs and exit shells multiple time.

Simple One-Liner Shell Scripts

Renames many files *.old to *.new. To test things first, replace ‘do mv’ with ‘do echo mv’.

for i in *.input; do mv $i ${i/\.old/\.new}; done
for i in *\ *; do mv "$i" "${i// /_}"; done # Replaces spaces in files by underscores
Run an application in loops on many input files.
for i in *.input; do ./application $i; done
Run fastacmd from BLAST program in loops on many *.input files and create corresponding *.out files.
for i in *.input; do fastacmd -d /data/../database_name -i $i > $i.out; done
Run SAM’s target99 on many input files.
for i in *.pep; do target99 -db /usr/../database_name -seed $i -out $i; done
Search in many files for a pattern and print occurrences together with file names.
for j in 0 1 2 3 4 5 6 7 8 9; do grep -iH <my_pattern> *$j.seq; done
Example of how to run an interactive application (tmpred) that asks for file name input/output.
for i in *.pep; do echo -e "$i\n\n17\n33\n\n\n" | ./tmpred $i > $i.out; done
Run BLAST2 for all *.fasa1/*.fasta2 file pairs in the order specified by file names and write results into one file.
for i in *.fasta1; do blast2 -p blastp -i $i -j ${i/_*fasta1/_*fasta2} >> my_out_file; done
This example uses two variables in a for loop. The content of the second variable gets specified in each loop by a replace function.
Runs BLAST2 in all-against-all mode and writes results into one file (‘-F F’ turns low-complexity filter off).
for i in *.fasta; do for j in *.fasta; do blast2 -p blastp -F F -i $i -j $j >> my_out_file; done; done;

How to write a real shell script

  • create file which contains in first line:
  • place shell commands in file
  • run <chmod +x my_shell_script> to make it executable
  • run shell script like this:
  • when you place it into /usr/local/bin you only type its name from any user account

Remote Copy: wget, scp, ncftp

wget: File Download from the Web

wget <ftp://> # file download from www; add option '-r' to download entire directories

scp: Secure Copy Between Machines

scp source target # Use form 'userid@machine_name' if your local & remote user ids are different. If they are the same, use only 'machine_name'.scp user@remote_host:file.name .Copies file from server to local machine (type from local machine prompt). The '.' copies to pwd, you can specify any directory, use wildcards to copy many files.
scp file.name user@remote_host:~/dir/newfile.name # Copies file from local machine to server.
scp -r user@remote_host:directory/ ~/dir  # Copies entire directory from server to local machine.

Nice FTP

 ncftp> open ftp.ncbi.nih.gov
 ncftp> cd /blast/executables
 ncftp> get blast.linux.tar.Z (skip extension: @)
 ncftp> bye

Archiving and Compressing

Creating Archives

tar -cvf my_file.tar mydir/    # Builds tar archive of files or directories. For directories, execute command in parent directory. Don't use absolute path.     
tar -czvf my_file.tgz mydir/   # Builds tar archive with compression of files or directories. For dirs, execute command in parent directory. 
zip -r mydir.zip mydir/        # Command to archive a directory (here mydir) with zip.
tar -jcvf mydir.tar.bz2 mydir/ # Creates *.tar.bz2 archive

Viewing Archives

tar -tvf my_file.tar
tar -tzvf my_file.tgz

Extracting Archives

tar -xvf my_file.tar
tar -xzvf my_file.tgz
gunzip my_file.tar.gz # or unzip my_file.zip, uncompress my_file.Z,or bunzip2 for file.tar.bz2
find -name '*.zip' | xargs -n 1 unzip # this command usually works for unzipping many files that were compressed under Windows
tar -jxvf mydir.tar.bz2 # Extracts *.tar.bz2 archive

Try also:

tar zxf blast.linux.tar.Z
tar xvzf file.tgz

Important options:

f: use archive file
p: preserve permissions
v: list files processed
x: exclude files listed in FILE
z: filter the archive through gzip

Simple Installs

Systems-wide installations

Installations for systems-wide usage are the responsibility of system administrator.  If you would like to have something installed, please contact us.

Environment Variables

xhost user@host                # adds X permissions for user on server.
echo $DISPLAY                  # shows current display settings
export DISPLAY=<local_IP>:0    # change environment variable
unsetenv DISPLAY               # removes display variable
env                            # prints all environment variables

List of directories that the shell will search when you type a command:

echo $PATH

You can edit your default DISPLAY setting for your account by adding it to file