Shell Script
============

One of the coolest part of UNIX command is that we can
script series of commands for automating tasks.

Example
-------

First, save following code into a file named "hello.sh".

.. code-block:: shell

  #!/bin/bash

  echo "hello world"

Afterwards, change the permission (adding executable permission)::

  $chmod u+x hello.sh

Now, let's execute the script::

  $ ./hello.sh
  hello world

If you modify the script as follows, output will also be changed:

.. code-block:: shell

  #!/bin/bash

  echo "hello world"
  echo "good morning"
  echo "good afternoon"
  echo "good evening"

Let's execute it again::

  $ ./hello.sh
  hello world
  good morning
  good afternoon
  good evening

The cool thing is for printing 5 lines, we do not need to type the echo command 5 times.

Exercise
--------

Let's build a shell script which computes the total size of files in a folder specified by the first argument of the script.

Expected output::

  $ ./filesize.sh DIRECTORY_NAME
  Total size of files in the directory is 8 KB

Hint 1: Getting argument

.. code-block:: shell

  #!/bin/bash
  
  echo "argument 1" $1
  echo "argument 2" $2

Hint 2: Getting the size of a file in KB::

  $ du -s FILE_NAME

Hint 3: Using loop

.. code-block:: shell

  #!/bin/bash

  i=0
  loop=3 # 3 times
  while [ $i -lt $loop ]; do
    echo "loop" $i
    i=$(($i+1))
  done

Compare number

=== ===
-eq ==
-ne !=
-lt <
-gt >
-le <=
-ge >=
=== ===

Hint 4: Printing files in a directory

.. code-block:: shell

  #!/bin/bash

  files=`ls`
  for f in ${files[@]}; do
    echo $f
  done

Hint 5: Arithmetic computation

.. code-block:: shell

  #!/bin/bash

  one=1
  two=2

  ret=$(($one+$two))
  echo "${one} + ${two} is ${ret}"

  ret=$(($one*$two))
  echo "${one} * ${two} is ${ret}"

Hint 6: Comparing string

.. code-block:: shell

  #!/bin/bash
  
  a1="hello world"
  a2="hello world"
  b="see you"
  
  if [ "$a1" = "$a2" ]; then
    echo "a1 and a2 are same"
  fi
  
  if [ "$a1" != "$a2" ]; then
    echo "a1 and a2 are different"
  else
    echo "a1 and a2 are same"
  fi
  
  if [ "$a1" != "$b" ]; then
    echo "a1 and b are different"
  fi

Hint 7: showing N-th column

.. code-block:: shell

  #!/bin/bash
  
  cat /proc/cpuinfo
  echo
  echo "----- only showing 1st column -----"
  echo
  cat /proc/cpuinfo | awk '{ print $1 }'

Example Answer

.. code-block:: shell

  #!/bin/bash
  
  kb=`du -s $1/* | awk '{ print $1 }'`
  
  total=0
  for k in ${kb[@]}; do
  	total=$(($total+$k))
  done
  
  echo "Total size of files in the directory is $total KB"