Lecture 3

• • Linux shell script is a text file with set execute permission (x). It may contain solely a sequence of shell commands or some additional control instructions making it more of a program structure.
  • Linux shell script is being interpreted by Linux shell, for example BASH shell or C shell.


  What is a Linux shell script?
• 1. Use any text editor to edit the script file.
  2. Change the created script file permission (default permissions = 644, should actually be 755).
  3. Copy the script file into one of the directories listed on PATH variable, can check this using echo $PATH
  4. Run the script file by typing in the command line.
  5. Script_file_name or ./script_file_name


  How to create a Linux shell script?
• 1. Keywords
  2. Special Variables
  3. System Variables

  Linux shell scripts
• • $0
  • The script name.
  • $1...$9
  • The command line argument.
  • $*
  • List of all command line arguments.
  • $@
  • Table of all command arguments.
  • $?
  • Status of the last executed command.
  • $#
  • Number of all arguments the script was executed with.

  Special Variables
• • SHELL
  • System shell
  • USER
  • User name
  • HOME
  • Home user directory
  • PS1
  • Shell command prompt
  • EDITOR
  • Default editor
  • PATH
  • Default directories list
  • MAIL
  • Default mail directory

  System Variables
• • Integer variables:
  • x = 3
  • String variables:
  • str = "a string" or str = 'a string'


  Types of variables
• • Variables do not have to be declared - first assignment of a value to a variable makes it declared.
  • x = 3
  • In order to refer to the variable a $ character has to be added at the beginning of the variable name.
  • Y = $X
  • The first line of the script is used to call bash shell for the remaining part of the script execution.
  • Calling the right shell at the beginning of the script guarantees that the remaining part of the script will execute correctly.


  Variables
• • Integer variables may be a subject of different arithmetic operations:
  • +, addition
  • -, subtraction
  • *, multiplication
  • /, division
  • %, division (modulo)
  • Arithmetic operations can be performed using two commands 'let' and 'expr' or $[] expression.

  Arithmetic operations
• • A table can be assigned as:
  • table = (value1 value2 ... valueN)
  • or
  • table[0] = 1
  • table[1] = 3 ...
  • table[N] = N
  • Tables do not have to be continuous.
  • To delete a selected table element the unset command is used:
  • unset table[2]
  • By calling unset table[*] or unset table[@] the table will be deleted as a whole.


  Tables
• • For, while, until
  • Loop instructions
  • If, case
  • Condition instructions
  • test
  • test instruction


  Control instructions
• If condition

  then

  command

  fi
• If condition

  then

  command1

  else

  command2

  fi
• If condition

  then

  command1

  elif condition

  command2

  else

  command3

  fi
• case $variable if

  value1) command1;

  value2) command2;

  ) command;;

  esac
• for VARIABLE in 1 2 3 4 5 ... N

  do

  command1

  command2

  commandN

  done
• while [condition]

  do

  command1

  command2

  command3

  done

  
  

  The condition is treated as a continuation condition unless the condition is false.

  While
• • test: logical test instruction
  • syntax: test condition
  • Test relations between integers.
  • Test relations between strings.
  • Test files information.

  Test
• • Relations between integers:
  • lt, less than, [$# -lt 2]
  • gt, greater than, [$a -gt $b]
  • le, lesser equal, [$a -le $1]
  • ge, greater equal, [$a -ge 2]
  • eq, equal, [$a -eq $b]
  • ne, not equal, [$c -ne 0]


  Test relations between integers
• • Relations between string:
  • ==, equal, ["$a" == "q"]
  • !=, not equal, ["$a" != "x"]
  • -n, empty, [-n "$a"]
  • -z, not empty, [-z "$1"]


  Test relations between strings
• • Files information:
  • -a, file exists.
  • -b, file exists and is a special block file.
  • -c, file exists and is a special character file.
  • -e, file exists.
  • -L, file exists and is a symbolic link.
  • -d, file exists and is a directory.
  • -r, read right set.
  • -w, write right set.
  • -x, execution right set.
  • -f, file exists and is an ordinary file.
  • -p, file exists and is a pipe.
  • -N, file exists and was modified since its last load.


  Test files information
• • In a Windows system all text files with '.bat' extension are assumed to be script files and can be executed.
  • The batch files may contain definitions of variables, control instructions, so they can be used for shell programming.

  Batch files
• • New variables can be instantiated like:
  • set name = value
  • The name must only be made up of alphabetic characters, numeric characters and underscores, it cannot begin with a numeric character.
  • Variables are referenced as:
  • %name%


  Variables
• • Command line arguments are treated as special variables within the batch script.
  • %0 = the name of the batch file itself.
  • %1 = the first argument.
  • %2 = the second argument.
  • To reference after the ninth argument you must use the shift command to shift the arguments 1 variable to the left.


  Command line arguments
• IF [NOT] ERRORLEVEL number command

  IF [NOT] string1 == string2 command

  IF [NOT] EXIST filename command

  • If you want to include more than one command, enclose it within brackets:

  @ echo off

  if exist hello.bat(

  echo Removing hello.bat...

  del hello.bat

  echo hello.bat removed!

  )

  Control Instructions - If
• FOR %%variable IN (set) DO command [command-parameters]

  @ echo off

  if exist bigtxt.txt rename bigtxt.txt bigtxt

  for %%f in (*.txt) do type %%f >> bigtxt

  rename bigtxt bigtxt.txt

  • A more useful option of the FOR construct is to use the option /L which turns in to a numeric mode whereby the following syntax applies.

  FOR /L %%variable IN (start,step,end) DO command [command-parameters].

  Control Instructions - For
• rem A Bang Counter

  @ echo off

  set n =% 1

  set i =

  : loop

  set i = %i%!

  echo Bang!

  if %i% == %n% goto end

  goto loop

  : end

  Control Instructions - Loops