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ULSTER COUNTY COMMUNITY COLLEGE
CIS 116 Linux (Part 2)
First Commands Karl Wick
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The Command Interpreter - Bash
From man: Bash is (a) … command language interpreter that executes commands read from the standard input (the keyboard) or from a file.
A blank character is used to separate words and redirections. The first word specifies the command to be executed. (The remaining words are treated as parameters.)
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Reserved Words
Reserved words have special meaning to the Bash shell;
case do elif else esac fi for function if in select then until while {} time [[ ]] ! | < >
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bash – Command Line Syntax
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bash – Rules of Syntax
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Naming Comventions
A file name may contain up to 256 characters. _, -, . are allowed. Other characters are also
allowed but not recommended because they may be OS command characters.
File name extensions are preceded by a dot (.) and may contain from 1 to 3 characters
Directory names may use extensions buy generally do not.
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Man(ual) q exits man
The most important Linux command is man pagename
man opens the manual for the page (or command specified).
Navigation:– <space> moves forward– <b> moves backward– <q> exits the manual
Type man man <enter>
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Print man pages
with man command | col -b | lpr Type !lpr filename <enter> (text pg 85)
Note: This only works if a printer is attached and configured.
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Swapping user account Remember that the # prompt means that you are
logged in as root. The $ prompt means that we are logged in as user.
Usually it is dangerous to be logged on as root, and sometimes it is essential.
We can switch user levels with the su command.su log into root with your own shellsu - log into root with its shell (better)su – name log into name accountsu – studentsu - root
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Where am I? pwd
With Linux’s extended tree hierarchy it is very easy to get lost.
The pwd command stands for “print working directory” and will tell you the complete directory path that you are in.– cd /etc cd sounds cd events
The command prompt shows only the lowest level directory. [root@localhost events]#
pwd shows: /etc/sound/events
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Changing the Active Directory
cd is the magic command– cd (alone) returns you to your home directory– cd with a path name will (may) take you there– There are two ways of specifying the path
• Absolute
• Relative
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Absolute vs Relative Pathnames A path is absolute if it specifies the entire
path name. An absolute path begins with / which is the name for the top directory.
A path is relative if it specifies where to go from the present directory
cd /etc/sounds/events is absolute if we are already in /etc we can type cd
sounds/events
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Some special paths
. Stay in current directory .. Move up one level … Move up two levels / is the top directory /home is the normal user directory /root is the root directory
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Options for the cd command
Command Function
cd return to login directory
cd ~ return to login directory
cd / go to system top directory
cd /root go to root directory
cd /home go to users home directory
cd .. move up one level
cd ~otheruser go to his login dir if permitted
cd /dir1/subdir absolute path
cd ../../dir3/X11 up two levels, then down to X11
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An experiment
Make sure that you are logged in as student (if not then type su – student) and you will
be. Type cd /root
– You will get a message “Permission denied” Change to root (su – root, then type pw) Now you are in the /root directory. The root directory is protected.
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List the directory with ls Type ls from the root directory You will see a few entries
– Blue means a folder– Grey means a file– Green means special items
Type cd / Type ls again You now see many folders here including root and
home Just like with DOS, ls may not show everything in
the directory.
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ls options
Type man ls When done type q– There are many options listed– We will use a few
These many options allow us to sort the information displayed to best meet our needs.
With options we can– Specify HOW files are displayed– See file permissions– See other file attributes
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ls options practice
Go back to the root directory cd Type ls again and note the output Now type ls –a and note the output again
– Are they the same?
You should see more files or folders. These inculde . and ..
All of the new files may begin with dots (.) These are hidden files (“dot” files). Try ls –A (all but implied {. and ..})
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Even more information
ls and ls –a give names only. We can also look at details. Use the l(ong)
option; ls –l Now we see a lot more information Many files are still missing. Try typing ls –al (this combines options)
– the output may scroll off of the screen– ls –al | more
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Useful ls Options Chart
-a all – list all files, including hidden
-l long – detailed: permissions, owner, etc.
-F file type - /=dir, @=link, *=executable
-r reverse – Back to front
-R Recursive – all dirs current and lower
-S Size – sorts files by size
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Finding files and directories
Sometimes you know that a file or directory exists but you cannot recall the exact location on the disk
The commands locate filename and slocate filename will find a file or directory.
slocate only lets you find files and folders that you have permission to access.
Format is locate [-options] <string> Example locate finger <enter>
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The cron daemon
The locate command finds names from a database created by a daemon called cron.
(Daemons handle background tasks) The database is called slocate cron updates the database nightly if the
system is running. If you shutdown after using the system cron rarely has a chance to update slocate.
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Updating slocate
As we’ve already mentioned, cron updates the slocate datebase nightly, but only if Linux is up.
We can force an update by entering the command updatedb <enter> at the root prompt (only root can run this utility).
The update takes a minute or two depending on the number of files and speed of the system.
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Clearing the Window
By now you must be getting tired of clutter filling your screen and the cursor appearing only at the very bottom.
Just like DOS, Linux has a clear screen command.
In DOS the command was CLS, In Linux the command is clear
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Listing the Contents of a File Remember all files are text files unless
otherwise specified In Linux, there are several ways displaying the
contents of a text file. The commands are;
cat is the most versatile and also has other useful functions. Sometimes this can get us into trouble.
cat
more less
head tail
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cat
cat is short for ‘concatenate’ – meaning ‘to connect files together in order’
cat will display an existing file cat will create a new file if the name is not
already taken cat uses the standard input and output
devices unless you specify otherwise.
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Standard Input & Output
The standard input device is the keyboard The standard output device is the monitor Type cat <enter>
– The cursor moves to a blank line
– Type “Stop at the music store.” <enter>
cat echoed the string to the screen To quit cat, move the cursor to a blank line and
press <CTRL-D>. The command prompt will reappear.
cat works a lot like the DOS ECHO command.
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Redirection
Redirection means directing the shell to get input or send output to/from somewhere other than the standard devices.
The > symbol redirects output The < symbol redirects input Placing > after any command that generated
output will send the output to that location (often a file)
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cat and Redirection Type cat > roadtrip.txt and then <enter> now type these lines (all followed by <enter>)
– Buy some CDs– Then visit the coffee shop– Buy some coffee– Go home, drink coffee and listen to CDs
At a blank line type <CTRL-D> The command prompt should reappear. The four lines that we typed did NOT appear on the
screen. Instead, they were sent to a file called roadtrip.txt
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cat with Redirection – continued
Type ls. you will see the new file listed. Read the file by typing cat roadtrip.txt
NOTE: Be careful using cat to create a new file because it will replace existing files with the same name without prompting.
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cat with Redirection – continued
Create another file called part2.txt cat > part2.txt <enter>
– Make supper– Watch the news – Do homework
<CTRL-D> ls cat part2.txt
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Concatenation with cat Let us now join our two files into a third file by
using the cat command cat roadtrip.txt part2.txt > saturday.txt ls cat saturday.txt What do you see on the screen? cat has appended the second file’s contents to the
first file. Format is cat file1 file2 … filen > destination cat can use any number of input files.
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Appending information
The special character > redirects information to a file
The special character >> appends information to the end of a file.
Type cat part2.txt >> roadtrip.txt cat roadtrip.txt roadtrip now contains its original text plus
the text of part2 at its end.
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Appending information
CAUTION!– Be careful to use the doubled >> when
appending information. If you accidentally use the single > you will replace your output file instead of adding information to it!
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Make a larger file
cat > longfile.txt Type the numbers 1 through 50 one to a
line. <ctrl-D> cat longfile.txt - The output scrolls off of
the screen. How can we display this file and see all of
it?
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more
The more command lets us display long files one page at a time with a pause for a keypress between pages.
Type more longfile.txt <enter>– At the bottom of the screen you see --More--(35%)
Press the enter key – the text moves up one line Press the space bar – the text moves up one page More is similar to the DOS command MORE You CANNOT scroll back up!
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less
less does what more does only more. less will display a file and will let you scroll
up and down with the arrow, page up, page down, b, enter and spacebar keys.
See man for more options. less longfile.txt <enter> While more exits automatically at the end
of a file, you must exit less by pressing the q key.
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head and tail head filename will display the first ten lines of a
file on the output device. tail filename will display the last ten lines of a file
on the output device. head longfile.txt <enter> tail longfile.txt <enter> These commands let us take a quick at the
beginning or ending of a file to see what kind of information it contains.
head –20 displays 20 lines, etc.
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Redirection with pipes
Pipes connect the standard output of one command to the standard input of another command.
Type ls –al /etc <enter> The information scrolls off of the screen Type ls –al /etc | less <enter> Remember all of those messages during the boot
process? Type dmesg | less to see them.
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Finding character strings
The grep command will find specific strings of characters within files
grep searches are case sensitive the –i option makes the search case
insensitive. (grep –i string filename) grep coffee saturday.txt will show every
line containing the word coffee. grep can redirect its output into a file for
later review grep buy part2.txt | review
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Wildcards * matches any string including null string ? matches any single character \* matches the * character \? matches the ? character […] matches any one of the enclosed characters. A
pair of characters separated by a dash indicates a range of characters.
[A-z] means match all letters If the first character is ! or ^ then any character
NOT enclosed is matched.
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wc - Word Count
wc [options] filename The generic version shows
– number of lines– number of words– number of bytes
wc longfile.txt
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diff - Comparing two files
The diff command compares two text files and finds the differences between them.
diff [option] file1 file2 – The –l option treats all letters as lower case– The –c option gives a very detailed comparison
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Command History
Linux stores a list of recently issued commands in a file called .bash_history. By default bash stores up to 500 (1000) commands.
You can review these commands in reverse order by typing the up arrow.
You can view the file with more, less, et.al. Typing env will show the size of the file.
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Tab Completion
You can type part of a simple command and then <tab> and bash will complete the command or beep.
If it beeps just hit tab again for a list of all commands beginning with what you typed.
m<tab><tab> mo<tab><tab>
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Multiple commands on a single line ; separates commands cat roadtrip.txt; cat saturday.txt will
display BOTH files on the screen.
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DOS/UNIX Cross Reference
DOS UNIX Function
DIR ls list directory contents
CD pwd show working directory
CD cd change working directory
MD mkdir make new directory
RD rmdir remove directory
DEL rm Remove file(s)
COPY cp copy file
REN mv move/rename file
TYPE cat display file (cat does more)
SORT sort sort file
MORE more display file with pause
CLS clear clear screen
DATE date Disp date (and time)
EDIT vi/emacs Built in editors
HELP man help files
VER uname –a OS info (and more)
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Special Key Combinations<CTRL-s> Stop screen output
<CTRL-q> Resume screen output
<CTRL-c> Stop current activity or process
<CTRL-d> Exit or End of File
<CTRL-w> Erase last word
<CTRL-u> Erase entire line
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Back to the regular book
Now that we have a good grasp of the basic operation of Linux let us
explore our text book.