Introduction to Unix (CA263) Decisions, Decisions By Tariq Ibn Aziz Dammam Community College

Objectives In this lecture you will learn –Statements that is present in almost all languages –Exit status –The $? Variable –The test command –File operators –The Logical Negation Operator ! –The Logical AND Operator -a

Objectives In this lecture you will learn –Parentheses –The Logical OR Operator –o –The else Construct –The exit Command –The elif Construct –The case Command –The Null Command –The && and || Construct

Exit status Whenever any program complete execution under UNIX system, it returns an exit status back to the system. This status is a number that usually indicates whether the program successfully ran or not. The exit status of zero indicates that a program ran successfully, and nonzero to indicate that it failed. Example: cp command return nonzero if the copy fails for some reason, or if arguments are not specified correctly.

Exit status Example In a pipeline, the exit status is that of the last command in the pipe. So in Who | grep fred The exit status of the grep is used by the shell as the exit status for the pipeline. In this case exit status of zero means that fred was found in who’s output.

The $? Variable The shell variable is automatically set by the shell to the exit status of the last command executed. $ cp phonebook phone2 $ echo $? 0 $ cp nosuch backup cp: cannot access nosuch $ echo $? 2 $ echo $? 0 $ who root console Jul 8 10:06 wilma tty02 Jul 8 1:11 betty tty11 Jul 8 1:10 $ Who|grep wilma wilma tty02 Jul 8 1:11 $ echo $? 0 $ Who|grep fred $ echo $? 1

if command $ cat on # # determine if someone is # logged on # user ="$1" if who | grep "$user" then echo "$user is logged on" fi $ $ who root console Jul 8 10:06 wilma tty02 Jul 8 1:11 tony tty03 Jul 8 1:12 betty tty11 Jul 8 1:10 joanne tty12 Jul 8 1:10 $ on tony tony tty03 Jul 8 1:12 tony is logged on $ on ann joanne tty12 Jul 8 1:10 ann is logged on

First problem in on program We found couple of problems, the first problem is, it displays the grep output. We can dispose of grep output by redirecting it to system “garbage can” /dev/null. who | grep "$user" > /dev/null

Second problem in on program The second problem with on appears when he program is executed with the argument ann. Even though ann is not logged on, grep matched the character ann for the user joanne. To solve this problem match the pattern to the beginning of the line by preceding character "^" who | grep "^$user" > /dev/null

Example $ cat on # # determine if someone is # logged on # user ="$1" if who | grep "^$user" >/dev/null then echo "$user is logged on" fi $ $ on ann $

The test Command A test command used to test one or more conditions in an if command. test expression test evaluate expression, if result is true, it return exit status of zero, otherwise nonzero. name=julio if test "$name" = julio then echo "Would you like to play a game? " fi

The test Command Even if name is null, the shell will still pass three arguments to test, the first one null #Set the name to null $ name= $ test "$name" = julio $ echo $? 1 $

The test string operators Returns TRUE (exit status of 0) if string1=string2, string1 is identical to string2 String1 != string2, string1 is not identical to string2 String, string is not null -n string, string is not null (a string must be seen by test ) -z string, string is null (a string must be seen by test )

Example test $ day="monday" $ test "$day" = monday $ echo $? 0 True $ $ day="monday " $ test "$day" = monday $ echo $? 1False $ $ day="monday " $ test $day = monday $ echo $? 0 True $ blank=" " $ test $blank it’s null $ echo $? 1 False $ test "$blank"it’s not Null $ echo $? 0 True

Example test -z test –z "$dataflag" will return and exit status of 0 if dataflag doesn’t contain any characters. $ nullvar= $ nonnullvar=abc $ test –z "nullvar" $ echo $? 0 $ test –z "nonnullvar" $ echo $? 1

Example test -n Does nullvar have nonzero length? NO $ nullvar= $ nonnullvar=abc $ test –n "nullvar" $ echo $? 1 Does nonnullvar have nonzero length? YES $ test –n "nonnullvar" $ echo $? 0

An Alternate Format of test test expression This can also be expressed in an alternate format as [ expression ] $ [ –z "nonnullvar" ] $ echo $? 1 if [ "$name" = julio ] then echo "How are you?" fi

Integer Operators int1 –eq int2, int1 is equal to int2 int1 –ge int2, int1 is greater than or equal to int2 int1 –gt int2, int1 is greater than int2 int1 –le int2, int1 is less than or equal to int2 int1 –lt int2, int1 is less than int2 int1 –ne int2, int1 is not equal to int2

Integer To see if $count variable has value equal to 0 (zero) [ "$count" –eq 0 ] [ "$choice" –lt 5 ] [ "$index" –ne "$max" ] tests to see if the value of index is not equal to the value of max. [ "$#" –ne 0 ] tests to see if the number of arguments passed to the command is not equal to 0.

Integer Example $X1="005" $X2=" 10"  String comparison $[ "x1" = 5 ] $echo $? 1'False  Integer comparison $[ "x1" -eq 5 ] $echo $? 0'True  String comparison $[ "x2" = 10 ] $echo $? 1'False  Integer comparison $[ "x2" -eq 10 ] $echo $? 0'True In first test, the string comparison operator = is used. In second test, the Integer comparison operator -eq is used. The third and fourth tests are similar, only in this case you can see how even a leading space stored in variable x2 can influence a test made with a string operator versus integer operator.

File Operators –d file, file is a directory –f file, file is an ordinary file –r file, file is readable by the process –s file, file has nonzero length –w file, file is writeable by the process –x file, file is executable

File Operator Example To see if file exist, and is an ordinary file [ -f /usr/steve/phonebook ] To see if file exist, and also readable file [ -r /usr/steve/phonebook ] To see if file contains at least one byte [ -s /usr/steve/phonebook ] if [ -s $ERRFILE ] then echo "Error found:" cat $ERRFILE fi

The Logical Negation Operator ! The urinary logical negation operator ! Can be placed in front of any other test expression to negate the result. [ ! –r /usr/steve/phonebook ] Will return zero exit status (true), if phonebook is not readable. [ ! –f "$mailfile" ] Will return zero exit status (true), if file specified by $mailfile does not exist. [ ! "$x1" = "$x2" ] Will return zero exit status (true), if $x1 is not identical to $x2. [ ! "$x1" = "$x2" ] same as [ "$x1" != "$x2" ]

The Logical AND Operator -a The operator –a performs a logical AND of two expressions and will return true only if both are true. [ -f "$mailfile" –a –r "$mailfile" ] Will return zero, if specified file is an ordinary file and is readable by you.

The Logical AND Operator -a The command [ "$count" –ge 0 –a "$count" –lt 10] Will return zero, if variable count contains an integer value that is greater than or equal to 0 but less than zero.

The Logical OR Operator -o The command [ -n "$mailopt" –o –r $HOME/mailfile ] This command will be true if the variable mailopt is not null or if the file $HOME/mailfile is readable by you. The –o operator has lower precedence than the –a operator "$a" –eq 0 –o "$b" –eq 2 –a "$c" –eq 10 will be evaluated by test as "$a" –eq 0 –o ("$b" –eq 2 –a "$c" –eq 10)

The else Construct A construct known as the else can be added to if command. $cat on # # determine if someone is logged on – version 3 # User= "$1" if who | grep "^$user" > /dev/null then echo "$user is logged on" else echo "$user is not logged on" fi

The else Construct Example $ who root tty02 Jul 7 08:37 fred tty03 Jul 8 08:30 tony tty04 Jul 8 08:17 lulu tty05 Jul 8 08:27 taziz tty06 Jul 8 08:57 ahmed tty07 Jul 8 08:47 $ on pat pat is not logged on $ on tony tony is logged on

The else Construct $ cat on # # determine if someone is logged on – version 4 # if [ "$#" –ne 1 ] then echo "Incorrect number of arguments" echo "Usage: on user" else user= "$1" if who | grep "^$user" > /dev/null echo "$user is logged on" else echo "$user is not logged on" fi

The else Construct $ on Incorrect number of arguments Usage: on user $ on priscilla priscilla is not logged on $ on jo anne Incorrect number of arguments Usage: on user

The exit Command The built-in shell command called exit enables you to immediately terminate execution of your shell program. exit n Where n is the exit status that you want to returned.

The exit Command Example $ cat rem # # remove someone to the phone book # if [ "$#" –ne 1 ] then echo "Incorrect number of arguments“ echo "Usage: rem name“ exit 1 fi grep –v "$1" phonebook >/tmp/phonebook mv /tmp/phonebook phonebook $ rem Susan Goldberg Incorrect number of arguments Usage: rem name The exit command will return 1 to signal failure

The elif command This sequence is used when you have multiple decision to make. $ cat greetings # # Program to print a greeting hour=`date|cut –c12-13` If [ "$hour" –ge 0 –a "$hour" –le 11 ] then echo "Good morning" elif [ "$hour" –ge 12 –a "$hour" –le 17 ] then echo "Good afternoon" else echo "Good evening" fi

The case command $ cat number # Translate a digit to English # if [ "$#" –ne 1 ] then echo "Usage: number digit" exit 1 fi case $1 in 0)echo zero;; 1)echo one;; 2)echo two;; 3)echo three;; 4)echo four;; *)echo Bad argument;; esac $ number 0 zero $ number 3 three $ number Usage: number digit $ number 5 Bad argument

The case command $ cat ctype # classify char as argument # if [ "$#" –ne 1 ] then echo "Usage: ctype char" exit 1 fi char="$1" numchar=`echo “$char”|wc –c` if [ "$numchars" –ne 1 ] then echo Single Character Pl. exit 1 fi case "$char" in [0-9] ) echo digit;; [a-z] ) echo lowercase;; [A-Z] ) echo uppercase;; *)echo Special Char;; esac $ $ ctype a lowercase $ ctype 7 Digit $ ctype abc Single Character Pl.

| Symbol $ cat greetings # # Program to print a greeting – case version # hour=`date %H` case "$hour" in 0? | 1[01]) echo "Good morning" 1[2-7]) echo "Good afternoon" *) echo "Good evening" esac $ date Fri Jul 19 15:07:48 EDT 1985 $ greetings Good afternoon

The Null Command : The shell built-in null command. : It is used to satisfy the requirement that a command appear particularly in if command. if grep "^$system " /usr/steve/systems>/dev/null then : else echo "$system is not a valid file system" exit 1 fi

The && and || construct The shell has two special constructs && and || that enable you to execute a command based upon whether the preceding command succeeds or fails. If you write command1 && command2 command1 will be executed and it returns an exit status of zero, then command2 will be executed. If command2 brought a nonzero exit status, then command2 get skipped.

&& Example Sort bigdata > /tmp/sort1 && mv /tmp/sort1 bigdata The mv command will be executed only if the sort is successful. Note that it is equivalent to writing if sort bigdata > /tmp/sort1 then mv /tmp/dort1 bigdata fi

The || constructs The || construct works similarly, only the second command gets executed only if the exit status of the first is non zero. So if you write grep "$name" phonebook || echo "couldn’t find $name" then the echo command will get executed if grep fails if grep "$name" phonebook then : else echo "Couldn’t find $name" fi

The || constructs You can write a pipeline on either the left or right-hand sides of these construct. On the left, then exit status is tested is that of the last command in the pipeline; thus who| grep "$n" >/dev/null|| echo "$n not logged on"