Chapter 3 Using Unix Commands

Chapter 3 Using Unix Commands Meenal K Jabde Modern College of Arts, Science & Commerce Ganeshkhind, Pune 16

Contents 1. What is Command. 2. Meaning of Internal and external commands. 3. The type command: knowing the type of a command and locating it. 4. The man command knowing more about Unix commands and using Unix online manual pages. The man with keyword option and whatis. 5. Knowing the user terminal, displaying its characteristics and setting characteristics. 6. The root login. Becoming the super user: su command. The /etc/passwd and /etc/shadow files. 7. Commands to add, modify and delete users.

What is command?  In computing, a command is a directive to a computer program to perform a specific task.  A command is a word or phrase that, when typed or spoken, causes the computer to perform a certain operation.

Internal and External Commands  The UNIX system is command-based i.e things happen because of the commands that you key in. All UNIX commands are seldom more than four characters long.  They are grouped into two categories:  Internal Commands : Commands which are built into the shell. For all the shell built-in commands, execution of the same is fast in the sense that the shell doesn’t have to search the given path for them in the PATH variable, and also no process needs to be spawned for executing it. Examples: source, cd, fg, etc.  External Commands : Commands which aren’t built into the shell. When an external command has to be executed, the shell looks for its path given in the PATH variable, and also a new process has to be spawned and the command gets executed. They are usually located in /bin or /usr/bin. For example, when you execute the “cat” command, which usually is at /usr/bin, the executable /usr/bin/cat gets executed. Examples: ls, cat etc.

How to find out whether a command is internal or external?  whether it is internal or external with the help of type command :  $type cat  cat is /bin/cat  //specifying that cat is  external type//  $type cd  cd is a shell builtin  //specifying that cd is  internal type//

man command  man, command in Linux is used to display the user manual of any command that we can run on the terminal.  It provides a detailed view of the command which includes NAME, SYNOPSIS, DESCRIPTION, OPTIONS, EXIT STATUS, RETURN VALUES, ERRORS, FILES, VERSIONS, EXAMPLES, AUTHORS  $man [OPTION]... [COMMAND NAME]...  1. No Option: It displays the whole manual of the command.

 2. Section-num: Since a manual is divided into multiple sections so this option is used to display only a specific section of a manual.  Syntax :  $ man [SECTION-NUM] [COMMAND NAME]  $ man 2 intro  In this example, the manual pages of command ‘intro‘ are returned which lies in the section 2.

 3. -f option: One may not be able to remember the sections in which a command is present. So this option gives the section in which the given command is present.  Syntax:  $ man -f [COMMAND NAME]  Example:  $ man -f ls  In this example, the command ‘ls‘ is returned with its section number.

 4. -a option: This option helps us to display all the available intro manual pages in succession.  Syntax:  $ man -a [COMMAND NAME]  Example:  $ man -a intro  In this example you can move through the manual pages(sections) i.e either reading(by pressing Enter) or skipping(by pressing ctrl+D) or exiting(by pressing ctrl+C).

 5. -k option: This option searches the given command as a regular expression in all the manuals and it returns the manual pages with the section number in which it is found.  Syntax:  $ man -k [COMMAND NAME]  Example:  $ man -k cd  The command ‘cd‘ is searched in all the manual pages by considering it as a regular expression.

 6. -w option: This option returns the location in which the manual page of a given command is present.  Syntax:  $ man -w [COMMAND NAME]  Example:  $ man -w ls  The location of command ‘ls‘ is returned.

 7. -I option: It considers the command as case sensitive.  Syntax:  $ man -I [COMMAND NAME]  Example:  $ man -I printf  The command ‘printf‘ is taken as case-sensitive i.e ‘printf‘ returns the manual pages but ‘Printf‘ gives error.

whatis command  whatis command in Linux is used to get a one-line manual page descriptions. In Linux, each manual page has some sort of description within it. So this command search for the manual pages names and show the manual page description of the specified filename or argument.  Syntax:  whatis [-dlv?V] [-r|-w] [-s list] [-m system[, …]] [-M path] [-L locale] [-C file] name …  whatis command with help option: The whatis command throws an error if no options, filename or arguments are passed. So, when we use the -h option, it gives the general syntax along with the various options that can be used.

1) whatis -d: This option prints the debugging information. Example: whatis -d ls 2) whatis -v: This option will prints verbose warning messages. Example: whatis -v ls 3) whatis -r : This option interprets each of the name as a regular expression. If any of the names match any part of a page name, a match will be made. This option causes the whatis command to be somewhat slower due to the nature of database searches. Example: whatis -r ls

4) whatis -w : This option interprets each name as a pattern containing shell style wildcards. For a match about to make, an expanded name needs to match the entire page name. This option causes the whatis command to be somewhat slower due to the nature of database searches. Example: whatis -w ls 5) whatis -l: This option does not trim output to the terminal width. Normally, the output will be truncated to the terminal width just to avoid ugly results from poorly- written NAME sections. Example: whatis -l cat 6) whatis -s: This option will going to search only the given manual sections. The list is a colon or a comma-separated list of sections. If an entry in list is just a simple section, for example, “3”, then all the displayed list of descriptions will include pages in sections “3”, “3perl”, “3x”, and so on; while if any of the entry in list has an extension, for example “3perl”, then only the list will include pages in that exact part of the manual section. Example: whatis -s 3 cat

Knowing the user terminal  1. Get the running processes of logged-in user using w  w command is used to show logged-in usernames and what they are doing. The information will be read from /var/run/utmp file. The output of the w command contains the following columns:  Name of the user  User’s machine number or tty number  Remote machine address  User’s Login time  Idle time (not usable time)  Time used by all processes attached to the tty (JCPU time)  Time used by the current process (PCPU time)  Command currently getting executed by the users

 Following options can be used for the w command:  -h Ignore the header information  -u Display the load average (uptime output)  -s Remove the JCPU, PCPU, and login time.

$ w  23:04:27 up 29 days, 7:51, 3 users, load average: 0.04, 0.06, 0.02 USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT ramesh pts/0 dev-db-server 22:57 8.00s 0.05s 0.01s sshd: ramesh [priv] jason pts/1 dev-db-server 23:01 2:53 0.01s 0.01s -bash john pts/2 dev-db-server 23:04 0.00s 0.00s 0.00s w $ w -h ramesh pts/0 dev-db-server 22:57 17:43 2.52s 0.01s sshd: ramesh [priv] jason pts/1 dev-db-server 23:01 20:28 0.01s 0.01s -bash john pts/2 dev-db-server 23:04 0.00s 0.03s 0.00s w -h

 $ w -u  23:22:06 up 29 days, 8:08, 3 users, load average: 0.00, 0.00, 0.00  USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT  ramesh pts/0 dev-db-server 22:57 17:47 2.52s 2.49s top  jason pts/1 dev-db-server 23:01 20:32 0.01s 0.01s -bash  john pts/2 dev-db-server 23:04 0.00s 0.03s 0.00s w -u  $ w -s  23:22:10 up 29 days, 8:08, 3 users, load average: 0.00, 0.00, 0.00  USER TTY FROM IDLE WHAT  ramesh pts/0 dev-db-server 17:51 sshd: ramesh [priv]  jason pts/1 dev-db-server 20:36 -bash  john pts/2 dev-db-server 1.00s w -s

2. Get the username and process of logged in user using who and users command who command is used to get the list of the usernames who are currently logged in. Output of the who command contains the following columns: user name, tty number, date and time, machine address. $ who ramesh pts/0 2009-03-28 22:57 (dev-db-server) jason pts/1 2009-03-28 23:01 (dev-db-server) john pts/2 2009-03-28 23:04 (dev-db-server)

 To get a list of all usernames that are currently logged in, use the following:  $ who | cut -d' ' -f1 | sort | uniq  john  jason  Ramesh  Users Command  users command is used to print the user name who are all currently logged in the current host. It is one of the command don’t have any option other than help and version. If the user using, ‘n’ number of terminals, the user name will shown in ‘n’ number of time in the output.  $ users  john jason ramesh

3. Get the username you are currently logged in using whoami  whoami command is used to print the loggedin user name.  $ whoami  john   whoami command gives the same output as id -un as shown below:  $ id -un  john

4. Get the user login history at any time  last command will give login history for a specific username. If we don’t give any argument for this command, it will list login history for all users. By default this information will read from /var/log/wtmp file. The output of this command contains the following columns:  User name  Tty device number  Login date and time  Logout time  Total working time $ last jason jason pts/0 dev-db-server Fri Mar 27 22:57 still logged in jason pts/0 dev-db-server Fri Mar 27 22:09 - 22:54 (00:45) jason pts/0 dev-db-server Wed Mar 25 19:58 - 22:26 (02:28) jason pts/1 dev-db-server Mon Mar 16 20:10 - 21:44 (01:33) jason pts/0 192.168.201.11 Fri Mar 13 08:35 - 16:46 (08:11) jason pts/1 192.168.201.12 Thu Mar 12 09:03 - 09:19 (00:15) jason pts/0 dev-db-server Wed Mar 11 20:11 - 20:50 (00:39

displaying its characteristics and setting characteristics.  stty command in Linux is used to change and print terminal line settings. Basically, this command shows or changes terminal characteristics.  Syntax:  stty [-F DEVICE | --file=DEVICE] [SETTING]...  stty [-F DEVICE | --file=DEVICE] [-a|--all]  stty [-F DEVICE | --file=DEVICE] [-g|--save]  Example: It will display the characteristics of the terminal

stty –all: This option print all current settings in human- readable form. stty --all

 stty -g: This option will print all current settings in a stty-readable form.  stty –g

 stty -F : This option will open and use the specified DEVICE instead of stdin.  Example:  stty -F D/  stty –help : This option will display this help and exit.  stty –help

 stty –version: This option will show the version information and exit.  stty --version

 Terminal Characteristics  You can specify your terminal type to Unix if the default is not suitable. To do so in the C shell, enter the command:  set term= (termtype)  where termtype is the name of a terminal type supported on the system. VT100 and VT220 are acceptable terminal types for example. If you always use the same kind of terminal, you may want to put this command in your .login.

Root login & su command  root is the username or account that by default has access to all commands and files on a Linux or other Unix-like operating system. It is also referred to as the root account, root user and the superuser su command The su command is use to change user ID or become super-user during a login session i.e. it allows you to become a super user or substitute user, spoof user, set user or switch user. Note: su only works if you know the target or root user’s password. su command syntax The syntax is: su - {user-name}

 If invoked without a user-name, su defaults to becoming the super user. The user will be prompted for a password, if appropriate.  To log in as root user type the following command (you need to supply root user account password when prompted):  $ su –  su Command Options  To display a list of commands, enter the following:  su –h

 Here are some common options to use with the su command:  Username – Replace username with the actual username you want to log in with. This can be any user, not just root.  –c or –command [command] – Runs a specific command as the specified user.  – or –l or –login [username] – Runs a login script to change to a specific username. You’ll need to enter a password for that user.  –s or –shell [shell] – Allows you to specify a different shell environment to run in.  –h or –help – Show the help file for the su command.  –p or ––preserve–environment – Preserve the shell environment (HOME, SHELL, USER, LOGNAME).

 Switch to a Different User  To switch the logged-in user in this terminal window, enter the following:  su –l [other_user]  You’ll be asked for a password. Enter it, and the login will change to that user.  If you omit a username, it will default to the root account. Now, the logged-in user can run all system commands. This will also change the home directory and path to executable files.  Use the whoami command to verify you switched to a different user.

The /etc/passwd and /etc/shadow files.  Users are accounts that can be used to login into a system.  Each user is identified by a unique identification number or UID by the system.  All the information of users in a system are stored in /etc/passwd file.  The hashed passwords for users are stored in /etc/shadow file.  When a new user is created, by default system takes following actions: 1. Assigns UID to the user. 2.Creates a home directory /home/. 3.Sets the default shell of the user to be /bin/sh. 4.Creates a private user group, named after the username itself. 5.Contents of /etc/skel are copied to the home directory of the new user. 6..bashrc, .bash_profile and .bash_logout are copied to the home directory of new user.These files provide environment variables for this user’s session.

contents of /etc/passwd File  This file is readable by any user but only root as read and write permissions for it. This file consists of the following colon separated information about users in a system:  Username field  Password field  An `x` in this field denotes that the encrypted password is stored in the /etc/shadow file.  The user ID number (UID)  User’s group ID number (GID)  Additional information field such as the full name of the user or comment (GECOS)  Absolute path of user’s home directory  Login shell of the user

 [username]:[password]:[UID]:[GID]:[GECOS]:[home_dir]:[shell_path]

contents of the /etc/shadow File  This file is readable and writable by only by root user. This file consists of the following colon separated information about password of users in a system:  User name field  Password field  Contains an encrypted password.  A blank entry, {:: }, indicates that a password is not required to login into that user’s account.  An asterisk, {:*:}, indicates the account has been disabled.  Last Password Change  This field denotes the number of days since the date of last password change counted since UNIX time (1-Jan-1970).  The minimum number of days after which the user can change his password.  Password validity

 Denotes the number of days after which the password will expire.  Warning period  Denotes the number of days before the password expiry date, from which the user will start receiving warning notification for password change.  Account validity  Denotes the number of days after which the account will be disabled, once the password is expired.  Account disability  This field denotes the number of days since which the account had been disabled counted from UNIX time (1-Jan-1970).

 Syntax:  [username]:[enc_pwd]:[last_pwd_change]:[pwd_validity]:[warn_date]:[acc_validi ty]:[acc_disablity]

Commands to add, modify and delete users  Add a user  To add users, run the useradd command, like so:  sudo useradd -m <name of the user>  sudo useradd -m john  By default, useradd creates a user without creating a home directory. So, to make useradd create a home folder, we’ve used the -m switch.

 Change Password of a User  Previously, when we created a new user, we’ve used the passwd command to assign a password to the new user. You can also use this to change passwords. If you want to change your own password, simply run:  Passwd  When you change your own password, it’ll ask you for your current password. Once you enter it correctly, you will be asked to enter your new password twice.

 You can also use it to change a different by providing the username, although you need to be root to change passwords for others. The syntax for changing another user’s password is:  sudo passwd <username>  When you are the root user, passwd doesn’t ask you for your current password — it’ll just ask you for the new password.

 Delete a User in Linux  To delete a user in Linux, you can use userdel command, like so:  sudo userdel <username>  By default, this command preserves the home directory and some other special files, such as the user’s list of cron jobs. If you want to delete these files as well, you should use the --remove-all-files flag.  For example, if you want to delete the user john along with the home directory of john, use the following command:  sudo userdel -r john

References 1) https://www.d.umn.edu/~gshute/unix/unix.html 2) https://www.tutorialspoint.com/unix_commands/refer.htm 3) https://www.guru99.com/must-know-linux-commands.html 4) http://mally.stanford.edu/~sr/computing/basic-unix.html 5) https://www.unixtutorial.org/basic-unix-commands

Chapter 3 Using Unix Commands

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