EXTRA - SCRIPT

SCRIPT:


KORN SHELL:
http://www.bolthole.com/solaris/ksh.html
http://www.well.ox.ac.uk/~johnb/comp/unix/ksh.html


SYMBOLS:
#!/usr/bin/ksh      <--this indicates the shell, which should be used to interpret the script

' '                 <--ignores all metacharacters between the quotes (echo '$HOME' --> $HOME) (with ssh variables are interpreted)
" "                 <--ignores all metacharacters except for $, ` and \ (echo "$HOME" --> /home/root) (with ssh everything is string-literal)
\                   <--ignores the special meaning of the following character (echo \$HOME --> $HOME)
                    (it is the continuation character as well, to continue a command in a new line)
${VAR}hello         <--{} distingueshes the variable from the surrounding text
~                   <-- represents the home directory of the user (~joe refer's to the home dir of joe)
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A single-quote cannot occur within single-quotes.

Some alternatives:
echo $'It\'s Shell Programming'  # ksh, bash, and zsh only, does not expand variables
echo "It's Shell Programming"   # all shells, expands variables
echo 'It'\''s Shell Programming' # all shells, single quote is outside the quotes
echo 'It'"'"'s Shell Programming' # all shells, single quote is inside double quotes
------------------------------------
x=day
echo there is a ${x}long meeting      <--{} is used to eliminate the need of space after the variable
there is a daylong meeting
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x=`date` or x=$(date)     <-() is supported only in ksh
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arithmeic:
(( x += 1 ))                 <- counter adds 1 to x
(( x -= 1 ))                 <- counter minus 1 from x
x=$(($x+1))                  <- counter adds 1 to x

let command can be used for this as well (+,-,*,/):
let x=x+10
let x=x/2

read <variable>              <- reads from the input to the given variable
export variable=value        <- passes variable into a subshell

set -x                       <- shows real time info for debugging
set -u                       <- unsets variables

test command:
The test command evaluates the expression and returns a return code 0 if true.
test expression or [ expression ] or [[ expression ]] (leave a space near the squaer braces)
In newer Korn shell the modern notation [[...]] is used, which is an extension of the test command.

integer operations:
if [ $? -eq 0 ] ; then       <- equal
if [[ $? -eq 1 ]]            <- [[ is for test condition
if [ $? -ne 0 ] ; then       <- not equal
if [ $? -lt 0 ] ; then       <- less than
if [ $? -gt 0 ] ; then       <- greater than
if [ $? -le 0 ] ; then       <- less equal
if [ $? -ge 0 ] ; then       <- greater equal
$number1 -eq $number2        <- numbers are equal
$number1 -ne $number2        <- numbers are not equal

string operations:
$string1 = $string2          <- strings are identical
$string1 !=  $string2        <- strings are not identical

if [[ -e $FILE ]]            <- if file exist

read command:
It reads one line from standard input and assigns the values  of each field to a shell variable.


print command (printf):
printf '%20s\n' "-- PLAYER --"

The syntax is %[-]m[.n]s
%s is the string, [-] is left justified, m is no. of chars wide, [n] is print the first n chars of the string.
The above example shows the string “- PLAYER -” printed, right justified in string wide of 20 chars.

VAR1="Hello"
VAR2="Worldddd"

printf '%10s %-10.5s\n' $VAR1 $VAR2
Hello World

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RUNNIG A SCRIPT:

Each shell script is executed in a subshell. Variables defined in a shell script cannot be passed back to the parent shell.

.                         <---if you invoke a shell script with a . (dot) it runs in the current shell.
                          Variables defined in this script are defined also for the current shell.

./                        <- refers to files in the current working directory
~                         <- refers to the home directory

3 ways of running a script:
ksh scriptname            (read permission needed)
scriptname                (read and execute permission needed)(in the PATH variable must be defined the dir)
. scriptname              (read permission needed) (it runs in the current shell, a subshell will not be invoked)


$$                        process id
$0                        shell script name currently executing
$1, $2 ...                parameters which were invoked with the script
$*                        equal all arguments passed
$#                        number of arguments passed into shell script (if [[ $# -ne 1 ]]...)
$?                        retrurn code of last command executed
command1 && command2      if command1 successful then do command2 (ls s* && rm s*) (if [[ $ACE = "HIGH" && $banker_score -ge 11 ]])
command1 || command2      if command1 not successful then do command2 ( if [[ $ACE = "HIGH" || $banker_score -ge 11 ]])

exit n                    pass a return code to shell script
$!                        process ID of last background process

true                      this command always returns a true result
false                     this command always returns a false result

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LOOPS:

for i in `cat file`
do
...
done


for i in /tmp/*            it will remove all files in /tmp
do
rm $i
done

while [[ $x -lt 9 ]]
do
...
done


read game?"Another Game (y|n)? "
      case $game in
            y|Y)  :;;            <- ":" means continue
            n|N)  EXT=1;;
      esac


--------------------------------------------------

CASE:
(it can happen that there is a problem with numerical variable so this is needed before the CASE condition: "x=echo $x")

case $x in
1 )
   echo
   ;;

2 )
   echo "Stopping the ovo agent..."
   ;;

* )
   echo "bye"
   ;;
esac

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TIPS & TRICKS:

cat <file> | head -n $x | tail -1            <-displays a line from a file
awk 'NR==2' /root/settitle                   <--displays the 2nd line in the file:/root/setitle
/usr/bin/echo yes | command                  <-- input is automatically yes
echo "$FS2:\t it is mounted now"             <--\t will do a tab in the output
perl -e 'select(undef,undef,undef,.3)'       <--it will sleep for 0.3 second

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Run the script every 2nd Sunday of the month:
DAY=$(date "+%d")
[ $DAY -ge 8 -a $DAY -le 14 ] || exit        <- if $DAY greater than 8 and lesser than 14 the script will continue
...rest of the script                        if not (|| means condition is false) it will exit


it will check if a file older than a week and if yes, delete it:
35 21 * * 0 /usr/bin/find /home/traffic/ -name "trace" -mtime +7 -exec /usr/bin/rm {} \\;


leaving the cursor at the same position while counting:
x=1
while [[ $x -le 100 ]]; do
   tput cub 20
   print -n $x
   (( x += 1 )
done