handout_2b.txt

CS 35L Software Construction Laboratory (Lab2-B)
Wed, April 11, 2012, Ver 1.1

Console, Shell, and Terminal
    Console: pure CLI
    Shell: a program emulate the console
           widely used shells: sh, bash, csh, tcsh
    Terminal: a gui based wrapper of the shell

Shell Scripting
    The first line starting with #! (shebang line or hashbang line)
      http://en.wikipedia.org/wiki/Shebang_(Unix)
      Tells the system which interpreter to interpret and execute the script.
      Makes shell scripts more like real excutable programs.
      eg. #! /bin/sh
          #! /usr/bin/python
    Variables
       In command line:  export A=23
       In scripts "export" can be omitted.
       Refer a variable: $A
       Variables hold string values.
       Quotation mark's function is to link two words as one.
       Output: "echo" or "printf"

    ---- sample1.sh -----
    # /bin/bash
    sum=0
    i=0
    while (( $i <= 10 )); do
        let sum=$sum+$i
        let i=$i+1
        echo $sum
    done
    ---------------------

Shell parameters
    The program itself is known as $0
    The first parameter to the shell is known as $1, the second as $2, etc. 
    The collection of ALL parameters is known as $*.
    ------------------------ sample2.sh  ------------------------
    #! /bin/bash
    
    printf "the program is: %s\n" $0
    printf "the first parameter is: %s\n" $1
    printf "the second parameter is: %s\n" $2
    printf "echo the collection of ALL parameters is: %s\n" "$*"
    -------------------------------------------------------------

How to execute a shell script?
    Two options:
    1) Make the file executable by adding "+x" attribute (chmod +x file_name)
    2) Call the bash interpreter directly (bash file_name)

More Linux Commands to learn:
  tr   -- transliterate files with a pattern
  sort -- sort lines of text files
  head -- display first lines of a file
  tail -- display the last part of a file
  comm -- select or reject lines common to two files
  cmp  -- compare two files byte by byte
  ln   -- make links
  grep -- print lines matching a pattern

Running in the background
    put an `&` at the end of the command/line of code
    Shell doesn't wait for the command to finish if the program is running
    in the background.

More examples

    -------------- sample3.sh -----------------
    This is how you can write an if-then-else
    control block
    -------------------------------------------
    #! /bin/bash
    VALID_PASSWORD=abcd1234
    echo "Please enter the password:"
    read PASSWORD
    if [ $PASSWORD == $VALID_PASSWORD ]; then
    	echo "You have access!"
    else
    	echo "ACCESS DENIED!"
    fi
    -------------------------------------------

    -------------- sample4.sh -----------------
    This is how you can write a 'for loop'
    -------------------------------------------
    #! /bin/bash
    for file in $(ls); do
        echo $file
    done
    -------------------------------------------