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Working with Linux
Here are some key points to keep in mind when working with Linux
Case sensitive: Linux considers uppercase letters to be different than lowercase letters. This means that the utility named cp is not the same as Cp or CP
To run a utility, always:
make sure you see the shell prompt first. The line you type your input is called the command line
start the command line with the command name
hit the enter key when you’re done
When you work with Linux, you interact with the shell
The shell works in a cycle:
prints the shell prompt to indicate that it’s ready for your input
interprets the command you type in
runs the corresponding utility that you request
sends the output to where you request
prints the shell prompt again
General Command Format
Commands that you type on the command line has the format:
command_name options arguments
The command_name is one word and is required
The options are optional. There can be 0, 1, or more options.
The arguments maybe required or optional, depending on the utility you select. There can be 0, 1, or more arguments.
The command_name, options, and arguments are separated by at least one space. Typically one space is used, but more than one space is okay.
More on command_name
The command name tells the shell which utility you want to run.
Each utility has a unique name. It is one short word or an abbreviation of a word.
The command name describes what the utility does
Examples: find, who, cp (for copy), ln (for link)
More on options
Without an option, the command works in the default mode.
For example, by default a command lists filenames in alphabetical order. With an option, the same command will list filenames in order of creation date.
Each option start with a + or – symbol, followed by one letter. Most options start with a – symbol.
When using multiple options, the options are separated by at least one space. Alternatively, you can put together multiple options with no space in between, and with one – symbol in front for options that start with a –. The same for + options.
With multiple options, the order you list the options is not important
Examples:
command ls with no option: ls
command ls with 1 option: ls –a
command ls with 2 options: ls –a –f
or ls –af
or ls –fa
More on arguments
Arguments are input to the utility. The utility performs its task on the input argument. For example: the copy utility will require 2 arguments: the source file and the destination file.
Depending on the utility, arguments are either required or optional
If using multiple arguments, list them in the order required or the order you want, separated by at least 1 space
Example:
Command ls with 1 argument: ls fileA
Command ls with 2 arguments: ls fileA fileB
Command ls with 2 arguments and 2 options: ls -af fileA fileB
Basic Commands
The basic commands covered in this section can be grouped into several common categories. They are commands dealing with:
The start and end of a session: passwd, exit
Information on users on the system: who, whoami, w, finger
Information about your system or your terminal: tty, stty, uname, clear
Getting labs done: man, lpr, script, ls, cat
General use: bc, date, cal, echo
exit
General Command Line Format
Commands to start and end a session
Commands to look up users
Commands to look up terminal and system information
Commands to get information on a utility
Commands to work with text files
Command to capture screen output
Calculator
Commands to look up time and day
Command to print text to screen
The exit command is used to exit out of a current process.
When you have successfully logged in to the system, the shell process starts running to wait for your command. When you type exit, the OS ends the shell process by closing the shell and you are logged out.
The time from when you log in up to when you log out is one session or one working session.
During a session you can start a second or third or more processes. Each time you run exit, you will get out of whatever current process you are in. So if you start 2 more processes from when you log in, you will need to run exit 3 times to completely log out.
The exit command sends an interrupt signal to the OS. Another way to send the interrupt signal is to use the combination of the control and d keys, abbreviated control-d
passwd
The passwd command allows you to change your password
Often in industry you are required to change your password on a regular basis for security reasons
System administrators set up the rules that your password needs to follow, such as at least 8 characters, or must be a combination of letters and non-letters, etc.
If your password doesn’t follow the rules, passwd will remind you of the rules
To run: passwd
Type in old password (security measure to prevent someone from maliciously changing your password on you)
Type in new password twice, once at each prompt
You should get a confirmation that it has been changed successfully
who, whoami, w, finger
Commands to find information on users who are in the system
who: gives you a snapshot of users who are currently logged in: user ID, terminal ID, date and location logged in
[testuser@abc ~]$ who
testuser pts/1 2012-09-03 12:37 (c-67-180-237)
testuser pts/2 2012-09-03 13:07 (c-67-180-237)
whoami: shows your user id only. Useful when you log in to multiple sessions with different user IDs
w: similar to who, but with more information such as the task the user is running
[testuser@abc ~]$ w
13:17:06 up 7 days, 21:15, 2 users, load average: 0.00, 0.00, 0.00
USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT
testuser pts/1 c-67-180-237-243 12:37 38:24 0.00s 0.00s man ls
testuser pts/2 c-67-180-237-243 13:07 0.00s 0.02s 0.00s w
finger: similar to who, but with more information such as the user’s name
[testuser@abc ~]$ finger
Login Name Tty Idle Login Time Office Office Phone
testuser Test User pts/1 42 Sep 3 12:37 (c-67-180-237)
testuser Test User pts/2 Sep 3 13:07 (c-67-180-237)
finger can also accept an argument of a user ID, or a last name, or first name. When given an argument, finger will find one or more users that match the argument and print more detailed information on each match.
Example: find all users with the name ‘testuser’
[testuser@abc ~]$ finger testuser
Login: testuser Name: Test User
Directory: /home/testuser Shell: /bin/bash
Never logged in.
Mail last read Wed Aug 27 16:13 20011 (PDT)
No Plan.
tty, stty, uname, clear
- Commands related to your terminal or system
tty: (terminal type) shows the ID of the terminal that you are logged in at
[testuser@abc ~]$ tty
/dev/pts/1
stty: shows basic settings of the terminal you are logged in at, -a option: shows all settings
[testuser@abc ~]$ stty
speed 38400 baud; line = 0;
-brkint -imaxbel
uname: shows basic system information
-a option: shows all system information
[testuser@abc ~]$ uname -a
Linux abc-desktop 2.6.18-92.1.10.el5 #1 SMP Wed Jul 23 03:56:11 EDT 2008 x86_64 x86_64 x86_64 GNU/Linux
clear: clears the screen
man
Each utility has a help page that explains how the utility works and lists all options for the utility
The help page is part of the online manual that is typically packaged with Linux, and the help page is commonly called the man page (short for manual page)
To see the man page for a utility: man utility_name
The required argument is the utility name
all information about the utility is shown one page at a time
To go to the next page: space bar
To go to the previous page: b (for back)
To get back to the shell prompt: q
lpr, ls, cat
- Commands to work with text files
lpr: (line printer) prints a file
Filename is the required argument
ls: (list) lists the filename of files you currently have
cat: (concatenate) shows the content of a file
Filename is required if you want to see the content of a specific file
script
script: captures output on screen into a file
Useful when you want to show proof of work, or show program output, or do error reporting by capturing the error on screen
When script runs, every character that appears on the screen also appears in an output file that you choose. When you exit out of script, the output file is saved and closed.
A filename is a recommended argument. If you don’t give a filename, a default filename (which is typescript) is used.
If you give the same filename as an existing file, the existing file will be overwritten
The –a option will cause script to append to an existing file, rather than overwrite it
Don’t run script when another script session is already running. This causes the output file to be very big and unreadable.
Don’t make too many typing mistakes when script is running. The mistaken characters and all the backspacing characters to fix the typing mistakes will all be recorded and appear as one long, ugly string in your output file.
Example of using script to capture screen output into a file called sample:
[testuser@abc ~]$ script sample <- start screen capture into file called sample
Script started, file is sample
[testuser@abc ~]$ who
tran pts/1 2012-09-03 14:05 (153.18.21.226)
testuser pts/2 2012-09-03 14:21 (c-67-180-237)
[testuser@abc ~]$ finger testUser
Login: testuser Name: Test User
Directory: /home/testuser Shell: /bin/bash
On since Wed Sep 3 14:21 (PDT) on pts/2 from c-67-180-237 (messages off)
No mail.
No Plan.
[testuser@abc ~]$ exit <- end screen capture
exit
Script done, file is sample
[testuser@abc ~]$ script -a sample <- start screen capture again
Script started, file is sample and append into file called sample
[testuser@abc ~]$ finger
Login Name Tty Idle Login Time Office Office Phone
testuser Test User pts/2 Sep 3 14:21 (c-67-180-237)
tran pts/1 15 Sep 3 14:05 (153.18.21.226)
[testuser@abc ~]$ exit <- end screen capture
exit
Script done, file is sample
Resulting sample file from the previous script sessions:
Script started on Wed 03 Sep 2008 02:21:32 PM PDT <- first script session
[testuser@abc ~]$ who
tran pts/1 2012-09-03 14:05 (153.18.21.226)
testuser pts/2 2012-09-03 14:21 (c-67-180-237)
[testuser@abc ~]$ finger test
Login: testuser Name: Test User
Directory: /home/testuser Shell: /bin/bash
On since Wed Sep 3 14:21 (PDT) on pts/2 from c-67-180-237 (messages off)
No mail.
No Plan.
[testuser@abc ~]$ exit
exit
Script done on Wed 03 Sep 2008 02:22:00 PM PDT
Script started on Wed 03 Sep 2008 02:22:22 PM PDT <- second script session
[testuser@abc ~]$ finger
Login Name Tty Idle Login Time Office Office Phone
testuser Test User pts/2 Sep 3 14:21 (c-67-180-237)
tran pts/1 15 Sep 3 14:05 (153.18.21.226)
[testuser@abc ~]$ exit
exit
Script done on Wed 03 Sep 2008 02:22:46 PM PDT
bc
bc: (basic calculator)
This calculator is actually not so basic, since it can support some programming (see the man page for bc). However in this class we will concentrate on basic arithmetic
To run: bc
To stop: quit
To do basic arithmetic, enter the arithmetic expression with or without space: 2 + 3 or 6/3
After the enter key, the result will appear on the next line
The basic operators: + (add), - (subtract), * (multiply), / (divide)
To change the number of digits after the decimal point: scale=n where n is the number of digits. Example: scale=2 for 2 digits after the decimal point, or scale=4 for 4 digits after the decimal point, or scale=0 for integers (no digit after decimal point)
date, cal
date: shows the current time and date
[testuser@abc ~]$ date
Wed Sep 3 20:18:13 PDT 2011
cal: shows the calendar
With no argument: shows the current month
With 1 argument: shows the year given by the argument, note that 2009 is not the same year as 09
With 2 arguments: the first argument is for the month (only values 1-12 are valid), the second argument is for the year
[testuser@abc ~]$ cal 2 2009
February 2009
Su Mo Tu We Th Fr Sa
1 2 3 4 5 6 7
8 9 10 11 12 13 14
15 16 17 18 19 20 21
22 23 24 25 26 27 28
echo
echo: prints to screen (echoes) the text that is given as argument(s)
Can accept one or many arguments
Arguments are text words for now
Useful to display a text string to screen, and in shell scripting, to send output of the script to screen
To run: echo any text string
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