Introduction to Unix CA263 Decisions, Decisions

1
Introduction to Unix (CA263) Decisions,
Decisions

• By
• Tariq Ibn Aziz
• Dammam Community College

2
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

3
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

4
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.

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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 whos output.

6
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 1006
• wilma tty02 Jul 8 111
• betty tty11 Jul 8 110
• Whogrep wilma
• wilma tty02 Jul 8 111
• echo ?
• 0
• Whogrep fred
• echo ?
• 1

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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 1006
• wilma tty02 Jul 8 111
• tony tty03 Jul 8 112
• betty tty11 Jul 8 110
• joanne tty12 Jul 8 110
• on tony
• tony tty03 Jul 8 112
• tony is logged on
• on ann
• joanne tty12 Jul 8 110
• ann is logged on

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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" gt /dev/null

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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" gt /dev/null

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Example

• cat on
• determine if someone is
• logged on
• user "1"
• if who grep "user" gt/dev/null
• then
• echo "user is logged on"
• fi
• on ann

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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.
• namejulio
• if test "name" julio
• then
• echo "Would you like to play a game? "
• fi

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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

13
The test string operators

• Returns TRUE (exit status of 0) if
• string1string2, 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)

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Example test

• day"monday"
• test "day" monday
• echo ?
• 0 True
• day"monday "
• test "day" monday
• echo ?
• 1 False

• day"monday "
• test day monday
• echo ?
• 0 True
• blank" "
• test blank its null
• echo ?
• 1 False
• test "blank" its not Null
• echo ?
• 0 True

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Example test -z

• test z "dataflag"
• will return and exit status of 0 if dataflag
  doesnt contain any characters.
• nullvar
• nonnullvarabc
• test z "nullvar"
• echo ?
• 0
• test z "nonnullvar"
• echo ?
• 1

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Example test -n

• Does nullvar have nonzero length? NO
• nullvar
• nonnullvarabc
• test n "nullvar"
• echo ?
• 1
• Does nonnullvar have nonzero length? YES
• test n "nonnullvar"
• echo ?
• 0

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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

18
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

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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.

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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.

21
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

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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

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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"

24
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.

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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.

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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)

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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" gt /dev/null
• then
• echo "user is logged on"
• else
• echo "user is not logged on"
• fi

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The else Construct Example

• who
• root tty02 Jul 7 0837
• fred tty03 Jul 8 0830
• tony tty04 Jul 8 0817
• lulu tty05 Jul 8 0827
• taziz tty06 Jul 8 0857
• ahmed tty07 Jul 8 0847
• on pat
• pat is not logged on
• on tony
• tony is logged on

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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" gt /dev/null
• echo "user is logged on"
• else
• echo "user is not logged on"
• fi
• fi

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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

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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.

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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 gt/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

33
The elif command

• This sequence is used when you have multiple
  decision to make.
• cat greetings
• Program to print a greeting
• hourdatecut 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

34
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

35
The case command

• cat ctype
• classify char as argument
• if "" ne 1
• then
• echo "Usage ctype char"
• exit 1
• fi
• char"1"
• numcharecho charwc 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.

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Symbol

• cat greetings
• Program to print a greeting case version
• hourdate H
• case "hour"
• in
• 0? 101 ) echo "Good morning"
• 12-7 ) echo "Good afternoon"
• ) echo "Good evening"
• esac
• date
• Fri Jul 19 150748 EDT 1985
• greetings
• Good afternoon

37
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/systemsgt/dev/null
• then
• else
• echo "system is not a valid file system"
• exit 1
• fi

38
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.

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Example

• Sort bigdata gt /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 gt /tmp/sort1
• then
• mv /tmp/dort1 bigdata
• fi

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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 "couldnt find
  name"
• then the echo command will get executed if grep
  fails
• if grep "name" phonebook
• then
• else
• echo "Couldnt find name"
• fi

41
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" gt/dev/null echo "n not logged
  on"