Compunet Corporation Introduction to Unix (CA263) Reading Data By Tariq Ibn Aziz Dammam Community College

Compunet Corporation Objectives In this lecture you will learn –how to read data –Special echo Escape characters –The $$ variable and temporary file –The exit status from read

Compunet Corporation read variables When read command is executed, the shell read a line from standard input and assign the first word read to the first variable listed in variables, the second word read to the second variables, and so on. read x y

Compunet Corporation read variables[2] $ read text Will read and store an entire line into the shell variable text.

Compunet Corporation Special echo Escape Characters The echo command always automatically displays a terminating newline character after the last argument. This can be suppressed if the last two character given to echo are the special escape character \c. Use echo –e to make effect of escape characters \b backspace \c Remove new line \f Formfeed \n NewLine \r Carriage return \t Tab character \\ Backslash character \nnn ASCII value is nnn

read Example[1] $cat rolo echo –e ' Would you like to: 1. Look up someone 2. Add someone to the phone book 3. Remove someone from the phone book Please select one of the above (1 – 3): \c ' read choice echo case "$choice" in 1)echo "Enter name to the look up:\c" read name lu "$name";;

Compunet Corporation read Example[2] 2) echo "Enter name to be added:\c" read name echo "enter number: \c" add "$name" "$number";; 3) echo "Enter name to be removed:\c" read name rem "$name";; *) echo "Bad Choice";; esac $

Compunet Corporation add program $ cat lu # # Look someone up in the phone book # grep "$1" phonebook

Compunet Corporation rem program $ cat rem # # remove someone to the phone book # grep –v "$1" phonebook >/tmp/phonebook mv /tmp/phonebook phonebook $

Compunet Corporation add program $ cat add # # add someone to the phone book version 2 # echo "$1$2" >> phonebook Sort –o phonebook phonebook $

Compunet Corporation The $$ Variable and Temporary Files If two or more people on your system use the rolo program at the same time, there’s a potential problem that may occur. In rem program problem may occur with the /tmp/phonebook. If more than one person is using rolo to remove an entry at the same time, phonebook file can get messed up.

Compunet Corporation The $$ Variable and Temporary Files $$ is equal to the process id number (PID) of your login shell. Each process on the UNIX system is given a unique process id number, using the value of $$ in the name of file will minimize the possibility of another process using the same file. grep –v "$1" phonebook >/tmp/phonebook$$ mv /tmp/phonebook$$ phonebook

Compunet Corporation The $$ Variable $ echo $$ 4668 $ ps PID TTY TIME COMMAND 4668 co 0:09 sh 6470 co 0:03 ps

Compunet Corporation Exit Status from read read always returns exit status of zero unless an end of the condition is detected on the input. Ctrl +d is the end of input from terminal If data is coming from a file, then it means that there’s no more data to read from the file.

Compunet Corporation Exit status Example[1] $ cat addi # add pairs of # integer while read n1 n2 do expr "$n1" + "$n2" done $ $ addi Ctrl-d $

Compunet Corporation Exit status Example[2] $ cat data $ addi sums $ $ cat sums $

Compunet Corporation Exit status Example[3] $ cat number lineno=1 cat $* | While read line do echo "$lineno. $line" lineno=`expr $lineno + 1` done $ $ cat phonebook Tariq877 Rashid816 Arshad884 $ number phonebook 1.Tariq Rashid816 3.Arshad884

Compunet Corporation Exit status Example[4] $ who | number 1.root console Jul 25 07:55 2.tariq tty03 Jul 25 07:55 3.rashid tty13 Jul 25 07:55 4.arshad tty04 Jul 25 07:55 $

Compunet Corporation Exit status Example[5] The number program don’t work well for lines that contains backslashes or leading whitespace characters. $ number Here are some backslashes:\ \* Here are some backslashes: * $ Leading whitespaces characters are removed from any line that’s read. The backslash characters are also interpreted by the shell when it reads the line.