CS 497C

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CS 497C Introduction to UNIXLecture 34 -
Shell Programming

• Chin-Chih Changchang_at_cs.twsu.edu

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Sample Scripts

• The script cpback.sh shown in Fig. 18.1 protects
  your files from accidental overwritting by the cp
  command.
• The script dentry1.sh shown in Fig. 18.2 accepts
  a code and its corresponding description from the
  terminal, performs some rudimentary validation
  checks, and then adds an entry to a file
  desig.lst.
• The continue statements let you reenter the data
  or start a fresh cycle.

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Sample Scripts

• The break statement in the inner loop quits the
  loop after adding the line.
• You should use _at_ with a for loop when using
  multiword arguments.
• for file in _at_
• basename is a command to strip directory and
  suffix from filenames
• basename hello.java .java
• hello

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Sample Scripts

• You can use basename inside a for loop to change
  the extensions of files.
• for file in .txt do
• leftnamebasename file .txt
• mv file leftname.doc
• done
• The cpback2.sh script accepts multiple filenames
  and backup them without overwritting.

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The Here Document (ltlt)

• The here document (ltlt) provides input to a script
  from the script itself.
• It can be used with both command substitution and
  variables.
• It is often used with commands that dont use a
  filename as argument or for running interactive
  programs noninteractively.
• mail cs497c ltlt EOF
• Test Date is date.
• EOF

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let Computation A Second Look (ksh and bash)

• Some advanced features of shell programming only
  happen in Korn and bash shell.
• Korn shell is located in /usr/bin/ksh. Bash shell
  is located in /bin/bash.
• You can compute with let statement
• let sum256128 echo sum
• There is no space after variable. If you need
  space, just quote the expression

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let Computation

• let sum3 6 4 / 2 echo sum
• x22 y28 z5 z((xyz)) echo z
• When a process is created by the shell, it makes
  certain features of its own environment to the
  child.
• The created process make use of the inherited
  parameters including
• The PID of the parent process.
• The UID (owner) and GUID (group owner) of the
  process.

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Sub-Shells

• The current working directory.
• The three standard files.
• Other open files used by the parent process.
• Some environment variables available in the
  parent process.
• A variable defined in the parent is visible in
  the child only when it is exported (export).
• However, when the child alters the value of the
  variable, the changed value is not seen by the
  parent.

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Sub-Shells Arrays

• The matching operators () run a group of commands
  in a sub-shell, but the dont spawn one.
• Korn and bash support one-dimensional arrays
  where the first element has the index 0.
• prompt2Enter your name
• echo prompt2

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Arrays

• You can use a space-delimited list enclosed with
  parentheses
• month_arr(0 31 28 31 30 31 30 31 31 30 31 30 31)
• echo month_arr6
• In the older version of Korn, you can use the set
  A statement
• set A mon_arr 0 31 28 31 30 31 30 31 31 30 31 30
  31
• Using the _at_ or as subscript, you an display all
  the elements of the array as well as the number
  of elements.

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Arrays String Handling

• echo month_arr_at_
• echo month_arr_at_
• The dateval.sh script in Fig. 19.2 use arrays to
  validate an entered date.
• The length of string can be found by preceding
  the variable name with a .
• nameJay Leno echo name
• You can extract a substring
• echo name44

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Conditional Parameter Substitution

• Shell variables can be evaluated in a conditional
  manner depending on whether they are assigned a
  non-empty value. This is called parameter
  substitution with the following format
• variablesymbol string where symbol can be ,
  -, or ?.
• When the option is used, variable is evaluated
  to string if it has a non-null value.

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Conditional Parameter Substitution

• foundls echo foundFiles found."
• When - option is used, variable is evaluated to
  string if it has a null value.
• The operator additionally assigns a value to
  the variable.
• while x1 -le 10
• The ? option prints an error message and exits
  the shell.
• grep pattern flname?No file .. quitting

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Shell Functions

• A shell function consists of a group of
  statements that are executed together as a bunch.
• Optionally, it also returns a value
• function_name()
• statements
• return value
• mainfunc.sh displays some functions.

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Shell Functions

• anymore()
• echo option "\n1 ?(y/n) \c" 1gt2
• read response
• case "response" in
• yY) echo 1gt2 return 0
• ) return 1
• esac
• anymore Wish to continue

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eval exec

• eval processes a command line twice and is used
  to simulate arrays and execute.
• With eval, you can create generalized numbered
  prompts and variables that significantly compact
  code.
• prompt1User Name
• x1 eval echo \promptx
• exec overlays the current shell when prefixed to
  a command.

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exec trap

• exec date
• To debug shell scripts, use set x at the
  beginning of the script so that every command
  line is echoed to the screen.
• Use trap if you want your script to respond to an
  interrupt in a specific way.
• trap command_list signal_list
• It is useful in removing temporary files when a
  script receives a signal.