Ceng 328 Operating System_ Lab4

Sed Statement

sed is basically a find and replace program. It reads text from standard input (e.g from a pipe) and writes the result to stdout (normally the screen).

The search pattern is a regular expression. This search pattern should not be confused with shell wildcard syntax.

To replace the string linuxfocus with LinuxFocus in a text file use:
cat text.file | sed 's/linuxfocus/LinuxFocus/' > newtext.file

This replaces the first occurance of the string linuxfocus in each line with LinuxFocus. If there are lines where linuxfocus appears several times and you want to replace all use:

cat text.file | sed 's/linuxfocus/LinuxFocus/g' > newtext.file

Cut Statement

Usage: cut [OPTION]... [FILE]...

Print selected parts of lines from each FILE to standard output.

 -b, --bytes=LIST    output only these bytes

 -c, --characters=LIST  output only these characters

 -d, --delimiter=DELIM  use DELIM instead of TAB for field delimiter

 -f, --fields=LIST    output only these fields; also print any line

              that contains no delimiter character, unless

              the -s option is specified

 -n           with -b: don't split multibyte characters

 -s, --only-delimited  do not print lines not containing delimiters

   --output-delimiter=STRING use STRING as the output delimiter

              the default is to use the input delimiter

   --help   display this help and exit

   --version output version information and exit

With no FILE, or when FILE is -, read standard input.

Examples:

1)  $cut –c 4-7 file2

2)  $cat > courses

      ceng112;ceng114;ceng102;

      ceng218;ceng212

      ^D

       $cut –f2 –d ‘;’ courses

        ceng114

        ceng212

      $

3) 
   a) # wc puts some space behind the output this is why we need sed:
      numofchar=`echo -n "Ceng328" | wc -c | sed 's/ //g' `
      # now cut out the last char
      rval=`echo -n "$1" | cut -b $numofchar`
   b) numofcharminus1=`expr $numofchar "-" 1`
      # now cut all but the last char:
      rval=`echo -n "$1" | cut -b 0-${numofcharminus1}`
Functions in Shell Script
functionname(){
     # inside the body $1 is the first argument given to the function
     # $2 the second ...
     body
}
You need to "declare" functions at the beginning of the script before you use them.

Example: Write Script to find out biggest number from given three numbers. Numbers are supplies as command line argument. Print error if sufficient arguments are not supplied.
help(){
  cat << HELP
  findBiggest  -- find the biggest of the three numbers
  USAGE: findBiggest #1 #2 #3
  EXAMPLE: findBiggest 12 5 33
  HELP
  exit 0
}
# we have less than 3 arguments. Print the help text:
if [ $# -lt 3 ] ; then
   help
else
   max=$1
   for i in $2 $3
   if test $i –gt $max
   then
      max=$i
   fi
   echo “Biggest number is $max”            
fi
Exercise1: Write a shell script that writes “I love operating system Lectures” into a file then ask user, the word that will be changed and the new word that will be replaced with the old one.
Ans:
echo –n “input file name:”; read inputfile
echo –n “output filename:”; read outputfile
echo “I love operating system lecture” > $inputfile
echo –n “Enter the word that you want to replace: “
read word1
echo –n “Enter the word that will be replaced with old one:“;
read word2
for I in ‘cat $inputfile’
do
    if test $i=$word1
    then
         echo –n $word2 >> $outputfile
    else
         echo –n $i >> $outputfile
    fi
    echo –n ‘’ >> $outputfile
done
echo –n “.” >> $outputfile
        
 
Exercise2: Write a shell script that renames multiple files. 
Ans:
OLD="$1"
NEW="$2"
# The shift command removes one argument from the list of
# command line arguments.
shift
shift
# $* contains now all the files:
for file in $*; do
    if [ -f "$file" ] ; then
      newfile=`echo "$file" | sed "s/${OLD}/${NEW}/g"`
      if [ -f "$newfile" ]; then
        echo "ERROR: $newfile exists already"
      else
        echo "renaming $file to $newfile ..."
        mv "$file" "$newfile"
      fi
    fi
done

Question: Write a shell script that converts the binary numbers into its decimals equivalents.