Introduction to Unix (CA263) Command File

1
Introduction to Unix (CA263) Command File

• By
• Tariq Ibn Aziz

2
Objectives

• In this lecture you will learn
• How to write your own commands and how to use
  shell variables.

3
Command File

• A shell program can be typed directly at the
  terminal
• who wc l
• Or it can be type into a file and then file can
  be executed by the shell
• cat gt nu
• Who wc -l

4
Execute the Command

• Type nu at the command name to the shell
• nu
• sh nu cannot execute
• You need to add execute permission to this nu
  script file

5
chmod Command

• Ls l nu
• rw-rw-rw- 1 taziz taziz 12 jul 10 1142 nu
• chmod x nu
• rwxrwxrw- 1 taziz taziz 12 jul 10 1142 nu
• Now try
• nu
• 8
• nu gt tally
• cat tally
• 8

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Another Example

• Suppose you are working on a proposal called
  sys.caps and followig command sequence is needed
  every time you want to generate a new copy of a
  proposal.
• tbl sys.caps nroff mm Tlp lp
• You can save it in a file and give it executable
  permission and execute script when you need.

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Another Example2

• cat run
• tbl sys.caps nroff mm Tlp lp
• chmod x run
• run
• Request id is laser1-15 (standard input)

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Another Example3

• cat stats
• date
• Who wc l
• pwd
• chmod x stats
• stats
• Wed Mar 23 115550 ETD 2008
• 13
• /home/aziz/documents/proposals

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Another Example3

• You can add some echo command to stats to make
  the output more informative.
• cat stats
• echo The current date and time is
• date
• echo
• echo The number of users on the system is
• who wc l
• echo
• echo Your current working directory is
• pwd

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Another Example4

• stats
• The current date and time is
• Wed Mar 23 115550 ETD 2008
• The number of users on the system is
• 13
• Your current working directory is
• /home/aziz/documents/proposals

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Comments

• A shell programming cannot be complete without
  comment statement.
• Whenever the shell encounters the special
  character at the state of a word, it takes as a
  comment and ignore the line.
• Here is an entire commentary line
• Who wc l count the number of users
• Test to see if the correct arguments were
• supplied

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Comments

• cat stats
• stats prints date, number of users
• logged on and current directory
• echo The current date and time is
• date
• echo
• echo The number of users on the system is
• who wc l
• echo
• echo Your current working directory is
• pwd

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Variables

• Like all programming languages, the shell allow
  you to store values into variables.
• A shell variable begins with an alphabetic or
  underscore (_) character, and followed by zero or
  more alphanumeric or underscore characters.
• variablevalue

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Variables

• To assign the value 1 to the shell variable count
• count1
• To assign the value /home/aziz/bin to the shell
  variable my_bin
• my_bin/home/aziz/bin

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Displaying the value of Variables

• The echo command is used to display the value
  that is stored inside a shell variable
• echo variable
• The is a special character to the shell. If a
  valid variable name follows the , then shell
  substitute the value stored inside the variable.
• echo count
• 1

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Displaying the value of Variables

• You can have the value of more than one variable
  substituted at a time.
• echo my_bin
• /home/aziz/bin
• echo my_bin count
• /home/aziz/bin 1

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Use of Variables value

• The value of variables can be used anywhere on
  the command line.
• ls my_bin
• mon
• nu
• textx
• pwd
• /home/aziz/documents/memos
• cd my_bin
• pwd
• /home/aziz/bin

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Use of Variables value 2

• number99
• echo There are number pens
• There are 99 pens

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Variables Example

• cat names
• Ziad
• Amir
• Salem
• Khalid
• commandsort
• command names
• Amir
• Khalid
• Salem
• Ziad

• commandwc
• option-l
• filenames
• command option file
• 7 names

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Variables Example2

• value110
• value2value1
• echo value2
• value1

• value110
• value2value1
• echo value2
• 10

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The Null Value

• Display the value of a variable that was never
  assigned
• echo nosuch
• You dont get an error message

• echo nosuch
• A variable that contains no value is said to
  contain the null value

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File Name Substitution and Variable

• Here is a puzzle for you
• x
• Will the shell store character into variable x,
  or will it store the names of all files in your
  current directory

• x
• echo x
• Addresses into names nu numbers stat phonebook

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File Name Substitution and Variable

• Was the list of files stored into the variable x
  when
• echo x
• was executed?

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File Name Substitution and Variable

• Shell does not perform file name substitution
  when assigning values to variables. Therefore,
• x
• Assigns the single character to x. This means
  shell did the file substitution when executing
  the echo command

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File Name Substitution and Variable

• echo x
• was executed as follows
• The shell scanned the line, substituting as the
  value of x
• The shell then rescanned the line, encountered
  and then substituted the name of the files in the
  current directory.
• The shell then initiated execution of echo
  passing it the file name as arguments

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The Variable Construct

• Suppose name of file is store in a variable
  filename. If you want to rename a file so that
  the new name was same as old, except with an X
  added to the end
• mv filename filenameX
• The shell thinks that filenameX is the full name
  of variable because it is a valid variable name.
  To avoid this problem use curly braces
• mv filename filenameX