CMT3321 0203

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CMT3321 (02/03)

• (Linux study guide 4 Shell scripting)
• For Lab. 4, 5
• Lecturer Mr. Lee Bo-sing (Joe)

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Agenda

• Programming basics
• Shell script basics
• Shell script content
• Shell quoting
• Shell variable
• Relational condition
• Logical operation
• Arithmetic statement
• Flow control statements
• Std file I/O statement
• Sed awk introduction ??

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Programming basics (1)

• What does a program/script do
• Input data (from keyboard or file)
• Process input data based on specification
• Output processed result (to monitor or file)
• 2 types of programs
• Compiled program
• Need to compile before running, eg. C program
• Executes faster
• Interpreted program
• Dont need to compile. An interpreter (shell) is
  responsible for interpret/translate it during
  running. eg shell script
• Executes slower
• Shell script is just a kind of interpreted program

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Programming basics (2)

• 2 types of programming bugs
• Syntax bug
• mainly due to unfamiliar with the programming
  language syntax linux environment. Lab 4, 5
  helps to develop your ability of shell scripting
  language syntax.
• Read the error messages carefully. It usually
  provides useful hints.
• Logical bug
• most difficult to discover
• Logical thinking error
• Careless human mistake

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Programming basics (3)

• General procedures for interpreted program
• Understand specification of your program from end
  user or your boss
• Design and write down your programming
  specification using any pseudo-code or flow-chart
• Edit source code of your program using any editor
  such as pico or vi or any editor you get used to
• chmod your source code to executable in unix
  environment. If you just type the name of script
  file, make sure that current directory (.) is
  included in the variable PATH. Or, type a
  relative or full pathname of your script file.
• Run and check for any conflict with
  specification.
• If conflict exists, modify your script file,
  debug and re-run. Repeat step 5, 6 until
  specification is fulfilled

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Shell script basics (1)

• How does UNIX know this is shell script?
• Kernel check the first 2 byte of line of script
  file for !
• Kernel then check whole first line for !/bin/ksh
  or !/bin/sh or !/bin/csh or !/bin/bash to
  determine which shell interpreter to use.
• Note that syntax of different shell varies. For
  consistency, I use K-shell for teaching. ??
• How to make shell script executable
• In unix prompt, chmod 700 script_filename
• Current Directory (.) is included in PATH

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Shell script basics (2)

• How to run shell script?
• use shell specified in your script to interpret
  your script (better as portable)
• Just type pathname of script
• Or use your current shell to interpret your
  script
• For ksh or sh script, . pathname_of_script
  (eg. . /.test.ksh)
• For csh script source Pathname_of_script (eg.
  source /.test.csh)
• Exit status
• 0 gt successful
• 1 gt fail

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Shell script content (1)

• A script can include any of followings
• First line MUST be !/bin/??
• Comments prefixed by
• Any shell built-in commands (mkdir, cd, echo,
  eval, etc.)
• Any UNIX external utilities or executable
  complied programs (cp, tar, grep, awk, etc.)
• Shell variable assignment statement
  (var_namevalue variable is declared and
  assigned at the same time)
• Read shell variable content by including
  VAR_NAME in script statement.

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Shell script content (2)

• Arithmetic or relational statement
• Control flow statement
• Conditional expression (if, then, else, elseif)
• Looping expression
• Case statement
• Test statement
• Std I/O statement
• File I/O statement
• Shell function definition calling

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Shell quoting (1)

• Some characters have special meaning to shell and
  need to be quoted when that special meaning is
  not wanted, eg. You want print .
• Special characters includes
• ( ) lt gt ? \
  newline tab space
• 3 types of quoting
• Quote single character by backslash (\) eg. Echo
  \. Output is
• Quote a string by a single quote ( ). All
  special meaning of quoted special characters is
  lost and these characters will be printed.
• Quote a string by a double quote ( ). Similar
  to single quote but some special characters such
  as , , \ , still have their special meanings.

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Shell quoting (2)

• When to use quoting
• Pass special meaning to the called program . eg.
  find . -name .rpm
• A string with white space
• Eg. echo There are space in the sentence
• Intend to print out special character.
• Eg. echo dollar sign is \. Back quote is \.
  Backslash is \\

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shell variable (1)

• Declaration and assignment within one statement
• In assignment statement, the variable is not
  prefixed with dollar sign. White space (space,
  tab) is not allowed on either side of the equal
  sign ()
• Eg. VAR abc (wrong)
• Eg. VARabc (correct)
• However, to access a variable, the variable must
  be prefixed with dollar sign.
• Eg. VAR-NAME (better) or VAR-NAME
• Eg. echo VAR_NAME is VAR_NAME
• Variable or environment variable defined in a
  script is not available after the scripts
  execution completes.
• Variables defined in sub-shell are localized and
  parent shell cannot access these variables
• Read and run file var1.ksh

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shell variable (2)

• Content of an variable is manipulated as string.
  Even when a variable contains a number, it is
  stored as a string of numeric character.
• Content can be
• single, double quoted for string
• backslash quoted for single character with
  special meaning , eg. ? ( )
  lt gt
• Eg. VAR1abc
• Eg. VAR2this is a variable
• Eg. VAR3today is date and VAR1 is VAR1
• Eg. VAR4\VAR1
• Read and run file var2.ksh

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shell variable (3)

• Special predefined variables
• ? gt exit status of last executed command
• gt process ID of current command
• 0, 1, 2, 3, 4, ..
• e.g. Command-name par1 par2 par3 par4
• 0 equals to command-name
• 1 equals to par1
• 2 equals to par2
• 3 equals to par3
• 4 equals to par4
• equals to number of parameters, i.e. 4
• Read and run file var3.ksh

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Relational condition

• expr1 -eq expr2
• Is equal to
• expr1 -ne expr2
• Is not equal to
• expr1 -lt expr2
• Is less than
• expr1 -le expr2
• Is less than or equal to
• expr1 -gt expr2
• Is greater than
• expr1 -ge expr2
• Is greater than or equal to
• Refer to file if1.ksh, if2.ksh

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Logical operation of conditions

• AND of 2 or more conditions
• Condition-1 condition-2
• expr1 -lt expr2 expr2 -gt
  expr3
• OR of 2 or more conditions
• Condition-1 condition-2
• expr1 -lt expr2 expr2 -gt
  expr3
• Logical NOT of a condition
• ! Condition
• ! expr1 -lt expr2 expr2 -gt
  expr3
• Refer to file if1.ksh, if2.ksh

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arithmetic

• Works for - / and any combination
• Syntax
• Let var-name34-32/2
• Read and run file arith.ksh

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Shell flow control statement

• Conditional expression (if, then, else, elseif)
• Case statement
• While loop statement
• For loop statement

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Conditional statement (1)

• if condition1
• then
• command_block1
• fi
• if condition1
• then
• command_block1
• else
• command_block2
• fi
• Read and run file if1.ksh

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Conditional statement (2)

• if condition1
• then
• command_block1
• elif condition2
• command_block2
• else
• command_block3
• fi
• Read and run file if2.ksh

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Case statement

• case string in
• pattern1) command_block1
• pattern2) command_block2
• patterni) command_blocki
• ) command_block
• esac
• Read and run file case.ksh

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While for loop statement

• While loop
• while condition
• do
• command_block
• done
• For loop
• for name in words
• do
• Command_block
• done
• Read and run file loop.ksh

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Std I/O

• Read data from stdin (keyboard) and assign this
  data to a variable by
• read VAR_NAME
• Print(or write) to stdout (monitor) by
• echo anything you want to print, can include
  variable content, space, execution result of a
  command

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file I/O (1) write to a file

• 3 steps
• Open a file and assign a file descriptor n to
  this file. This file is created if not existed.
  If already created, all original content will be
  erased.
• exec ngtfilename (n gt 3)
• Redirect any std output of a command to this open
  file (file descriptor n) using gt
• echo message 1gtn
• Close the file
• exec ngt-
• Above 3 steps can be simplified into
• echo message gt filename

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file I/O (2) - Append to an existed file

• 3 steps
• Open the file and assign a file descriptor to
  this file.
• exec ngtgtfilename (n gt 3)
• Redirect any stdout of a command to this open
  file (file descriptor n) using gtgt
• echo message 1gtgtn
• Close the file
• exec ngt-
• Above 3 steps can be simplified into
• echo message gtgt filename
• Read run file1.ksh

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file I/O (3) read from an existed file

• 3 steps
• Open existed file for reading and assign a file
  descriptor to this file.
• exec nltfilename (n gt 3)
• Redirect any std input of a command to read from
  this open file (file descriptor n)
• while read LINE 0ltn
• do
• ..
• done
• Close the file
• exec nlt-
• Read run file2.ksh

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Sed

• Function works on stdin or file. Very useful
  tool
• to replace any src_string from stdin to
  dest_string and print to monitor by s
• eg. cat file sed -e s/src_str/dest_str/g
• s stands for substitute,
• g stands for global,
• if g is not given, only first src_str is replaced
  by dest_str.
• If g is given, all src_src of each line are
  replaced by dest_str
• No limitation to length of src_str and dest_str
• To delete particular target_str (just specify
  dest_str as nothing by s
• Eg. cat file sed -e s/target_str//g

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Sed

• To delete any line containing any target_str by
  d
• cat filename sed -e /target_str/d
• d stands for delete
• To delete any empty line by d
• cat filename sed -e //d
• To delete particular line(s) by d
• cat filename sed -e 1,4d (line 1 to
  4)
• cat filename sed -e 1d (line 1
  only)
• cat filename sed e d (last
  line)
• cat filename sed -e 2,d
  (2nd to last line)
• To select particular line(s) by p -n
• cat filename sed -n 1,4p
  (line 1 to 4)
• cat filename sed -n 4p
  (line 4 only)

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Sed

• More precisely to specify the src_str, target_str
  by
• Eg. cat file sed -e s/src_str/dest_str/
  g
• src_str stands for only src_str at the
  beginning of each line
• Eg. cat file sed -e s/src_str\/dest_str/g
  
• src_str\ stands for any src_str at the end of
  each line (\ is used as quote special meaning
  of to shell)
• More on sed from http//www.ptug.org/sed/sedfaq.ht
  m

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awk

• Actually a complete programming language. You can
  make use its column selection feature to write
  your script
• Similar to cut
• Eg. To select field 1,2,3 (field starts counting
  at 1) , separated by , from a file
• cat filename awk -F print 1,2,7
• -F specify as field separator, output field
  1, 2, 7
• More on awk from http//www.ssc.com/ssc/eap/

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• Open
• Discussions

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Exercise 1

• Run understand each of demo. Files
• var1.ksh, var2.ksh, var3.ksh
• if1.ksh, if2.ksh
• arith.ksh
• loop.ksh
• case.ksh
• file1.ksh file2.ksh

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Exercise 2

• Write a script which display Good morning,
  good afternoon or good evening , depending on
  the time of running the script, to monitor when
  it runs.

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Exercise 3

• Write a script which
• a. prompt to read your name age
• Enter your name Joe
• Enter your age 25
• b. write the read name to first line of file,
  named my_info.
• c. change your age by age2-3 and write to
  second line of my_info.
• Content of my_info
• my name is Joe
• my modified age is 72

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Exercise 4

• Write a script which read a number in units of
  seconds and convert it hourminutesecond and
  print to monitor
• Your script must prompt for re-input if negative
  value is input
• Enter number of seconds 12345
• Result
• 12345 seconds is 32545

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Exercise 5

• Write a script calculate, which accepts 4
  arguments a, b, c, d and print out a20-b2c/d
  to monitor
• calculate 2 12 5 2
• result of 220 122 5/2 is 18