A Quick Reference 

Unix commands are the first thing needed by a unix  sysadmin  who are  starting to work in unix operating system . Unix operating systems  comes with online manual  system which can be used to see the command details , syntax options and examples on while working on a unix  system. Unix manual can be accessed using man <command name> and it requires the man package   installed and MANPATH  set to man  directories. The manual page directories may differ in different unix operating systems and  man package may not be installed in all systems .

Following are a few of the most popular and useful commands used in unix operating system

The * wildcard character substitutes for one or more characters in a filename. For instance, to list all the files in your directory that end with .c, enter the command

ls *.c

? (question mark) serves as  wildcard character for any one character in a filename. For instance, if you have files named prog1, prog2, prog3, and prog3  in your directory, the Unix command:

ls prog?

-u is userid , if not specified system takes highest available .

-g group id should be existing in /etc/group , if not specified other or user is assigned.

-d home directory , default is to use user as the directory name under the home directory.

loginname - new login name to be created .

#useradd testlogin    will create a user by the name 'testlogin' with all default values .

Telnet brings up the login prompt of remote host and  expects you to enter your user name & password .Without argument it enters command mode (telnet>) and accepts command listed by ? at telnet> prompt.
Communication is not encrypted between two hosts.

ssh  username@hostname  or ssh -l username hostname

Depending on ssh setting for your account you may or may not be asked a password to login. Your login/passwd will be same login password as you would use with telnet connection.

Communication is encrypted between two hosts so if someone intercepts your communication he will not be able to use it.

ftp hostname

ftp expects you to enter  your username/passwd or if it is ftp only account it will require ftp account password .

put , mput (multipleput) command is used to transfer files to remote host.

get , mget (multipleput) command is used to transfer files from remote host.

ftp allows some limited number of commands to be executed at ftp> prompt & summary of ftp command can be found by using ? at ftp>  prompt

Communication is encrypted between two hosts.

ping hostname

If you can ping a host the host is reachable from the machine that you are using .

Router/firewall configuration may prevent ping to succeed .

tar tvf filename.tar   ---  View the table of content of a tar archive

tar xvf filename.tar   --- Extract content of a tar archive

tar cvf filename.tar  file1 file2  file3 --- Create a tar archive called filename.tar using file1, file2,file3 .

tar can't copy the special files , device files .Not suitable for taking root backup.

cpio is mostly used in conjunction with other commands to generate a list of files to be copied :
#ls | cpio -o > /dev/rmt/c0t0d0 -- Copy the contents of a directory into a tape archive:
#find . -depth -print | cpio -pd newdir --- copy entire directory to other place:
#find . -cpio /dev/rmt/c0t0d0 --- Copy files in current directory to a tape

cpio can copy special files and hence useful in taking root backup containing device file.

Find  command is used to find the files , directories and to run commands on the list of files thus generated .By default, find does not follow symbolic links.

 

find . -name *.log -print    --- Simple find to list log files

find . -name '*.log' -exec rm  {} \;  -- Simple find to find log files and delete them .

find accepts a long list of options to find the files based on different parameters such as create time , modified time , of certain size etc. Please refer to man find for more option.