ULSTER COUNTY COMMUNITY COLLEGE

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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
• ! lt gt

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bash Command Line Syntax
5
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
• ltspacegt moves forward
• ltbgt moves backward
• ltqgt exits the manual
• Type man man ltentergt

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Print man pages

• with man command col -b lpr
• Type !lpr filename ltentergt (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 shell
• su - log into root with its shell (better)
• su name log into name account
• su student
• su - root

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Where am I? pwd

• With Linuxs 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_at_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, _at_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 ltstringgt
• Example locate finger ltentergt

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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 weve already mentioned, cron updates the
  slocate datebase nightly, but only if Linux is
  up.
• We can force an update by entering the command
  updatedb ltentergt 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 ltentergt
• The cursor moves to a blank line
• Type Stop at the music store. ltentergt
• cat echoed the string to the screen
• To quit cat, move the cursor to a blank line and
  press ltCTRL-Dgt. 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 gt symbol redirects output
• The lt symbol redirects input
• Placing gt 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 gt roadtrip.txt and then ltentergt
• now type these lines (all followed by ltentergt)
• Buy some CDs
• Then visit the coffee shop
• Buy some coffee
• Go home, drink coffee and listen to CDs
• At a blank line type ltCTRL-Dgt
• 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 gt part2.txt ltentergt
• Make supper
• Watch the news
• Do homework
• ltCTRL-Dgt
• 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 gt saturday.txt
• ls
• cat saturday.txt
• What do you see on the screen?
• cat has appended the second files contents to
  the first file.
• Format is cat file1 file2 filen gt destination
• cat can use any number of input files.

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Appending information

• The special character gt redirects information to
  a file
• The special character gtgt appends information to
  the end of a file.
• Type cat part2.txt gtgt 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 gtgt when appending
  information. If you accidentally use the single gt
  you will replace your output file instead of
  adding information to it!

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Make a larger file

• cat gt longfile.txt
• Type the numbers 1 through 50 one to a line.
• ltctrl-Dgt
• 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 ltentergt
• 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 ltentergt
• 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 ltentergt
• tail longfile.txt ltentergt
• 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 ltentergt
• The information scrolls off of the screen
• Type ls al /etc less ltentergt
• 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
  lttabgt 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.
• mlttabgtlttabgt
• molttabgtlttabgt

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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
ltCTRL-sgt Stop screen output
ltCTRL-qgt Resume screen output
ltCTRL-cgt Stop current activity or process
ltCTRL-dgt Exit or End of File
ltCTRL-wgt Erase last word
ltCTRL-ugt 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.