CS 497C Introduction to UNIX Lecture 33: Shell Programming

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CS 497C Introduction to UNIXLecture 33 -
Shell Programming

• Chin-Chih Changchang_at_cs.twsu.edu

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The if Conditional

• The if statement takes two-way decisions
  depending on the fulfillment of a certain
  condition.
• There are three forms of the if statement
• if condition then commands else commands fi
• if condition then commands fi
• if condition then commands
• elif condition then commands else commands fi
• emp3.sh firewall

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The if Conditional

• emp3.sh mail
• emp3a.sh firewall
• emp3a.sh mail
• We saw two forms of the if conditional
  if-then-fi and if-then-else-fi. Theres a third
  form if-then-elif-then-else-fi.
• cronfind.sh find /

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Test and Companions of if

• When you utilize if to evaluate expressions, the
  test statement is often used as its control
  command.
• test ueses certain operators to evaluate the
  condition on its right and returns either a true
  or false exit status, test works in three ways
• Compare two numbers.
• Compare two strings or a single one for null
  value.
• Checks a files attributes.

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Test and Companions of if

• Test doesnt display any output but simply
  returns a value that sets the parameter ?.
• The numerical comparison operators used by test
  are
• -eq equal to
• -ne not equal to
• -gt greater than
• -ge greater than or equal to
• -lt less than
• -le less than or equal to

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Test and Companions of if

• cat arg_number_check.sh
• !/bin/sh
• if test -ne 3 then
• echo "You didn't enter three arguments"
• else
• echo "You entered the right number"
• fi
• The test statement has a shorthand a pair of
  rectangular brackets.

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Test and Companions of if

• The following two forms are equivalent
• test x eq y
• x eq y
• Test can be used to compare strings with the
  following operations
• s1 s2 true if s1 s2
• s1 ! s2 true if s1 is not equal to s2
• stg true if stg is assigned and not null
• -n stg true if stg is not a null string

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Test and Companions of if

• -z stg true if stg is a null string
• s1 s2 true if s1 s2 (korn and bash only)
• The compile.sh script is used to compile the last
  modified c or java programs.
• test can be used to test various file attributes
  (Page 554 Table 18.4)
• -f fname fname exists and is a regular file.
• -f fname finame exists and is readable.
• -w fname fname exists and is writable.

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Test and Companions of if

• !/bin/sh
• if -eq 1 then
• if 1 "j" then
• filels -t .java head -1
• javac file
• elif 1 "c" then
• filels -t .c head -1
• cc file a.out
• else
• echo "Invalid file type"
• fi
• else
• echo "Usage 0 file_type\nValid file types
  are c and j"
• fi

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Test and Companions of if

• -x fname fname exists and is executable.
• -d fname finame exists and is a directory.
• The ! negates a test, so ! w file negates -w
  file.
• Check the file filetest.sh.
• filetest.sh emp3.lst
• filetest.sh emp.lst

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The case Conditional

• case is a compact string-matching construct and
  is closed with esac.
• It uses the shells wild cards to match multiple
  patterns in egrep-style.
• The when used as the last option matches
  everything not matched by the previous options.
  The wild cards match strings and not files.
• case is specially suitable for matching the
  filename 0.

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The case Conditional

• Here is its syntax
• case expression in
• pattern1) commands1
• pattern2) commands2
• .
• esac
• case first matches expression with pattern1. If
  the match succeeds, then it executes commands1.
  Otherwise, then go to pattern2.

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The case Conditional

• !/bin/sh
• tput clear
• echo "\n 1. Find files modified in last 24
  hours\n 2. The free disk space\n 3. Space
  consumed by this user\n 4. Exit\n\n SELECTION
  \c"
• read choice
• case choice in
• 1) find HOME -mtime -1 -print
• 2) df
• 3) du -s HOME
• 4) exit
• ) echo "Invalid option"
• esac

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Expr Computation and String Handling

• expr is used for integer computation and string
  manipulation. It is used with the Bourne shell
  for incrementing the value of a variable.
• This command combines two functions in one
• Performs arithmetic operations on integers.
• Manipulates strings.

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Expr Computation and String Handling

• The (add), - (subtract), (multiply), /
  (divide), and (divide and truncate the decimal
  portion).
• x3 y5 expr x y
• expr uses regular expressions to extract a
  substring, locate the position of a character,
  and evaluate the length of a string.
• Korn and bash shells dont need expr.
• sleep specifies the number of seconds for which
  the shell will pause.

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While and until Looping

• The while loop executes its body as long as the
  control command returns a true value.
• It is used in scripts that repeatedly increment
  the value of a variable or provide multiple
  chances to a user.
• You can set up an infinite loop using true as the
  control command.
• The until loop complements while.

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While and until Looping

• The syntax of the while command is
• while condition is true
• do
• commands
• done
• while ! r invoice.lst
• do
• sleep 60
• done

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While and until Looping

• until r invoice.lst
• do
• sleep 60
• done
• for works with each element of a list at a time.
  The list can be generated by variables, wild
  cards, positional parameters and command
  substitution.

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for Looping with a List

• The syntax of this construct is as follows
• for variable in list do
• commands
• done
• for file in chap do
• cp file file.bak
• echo file copied to file.bak
• done

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for Looping with a List

• for file in .c do
• cc -o file.o file
• done
• All loops use the keywords do and done.
• The break statement terminates a loop, while
  continue starts the next iteration.