Day 8,9,10 COP 3502 Introduction to Shell Scripts

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Day 8,9,10COP 3502 Introduction to Shell Scripts

• David A. Gaitros
• Department of Computer Science
• Florida State University

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What is a shell script?

• A group of UNIX commands along with a few
  programming constructs placed in a file that is
  executable. Must like a program.
• Used when a function needs to be completed over
  and over again.
• Your startup scripts are shell scripts.

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Simple Shell Scripts

• The following text is placed in a file called
  simple
• A Comment starts with an
• cal
• date
• who

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How to run a shell script

• There are two ways
• 1. Use the (sh) command
• sh simple ltrgt
• 2. Change the permissions to executable and just
  enter the file name
• chmod 755 simple ltrgt
• chmod urwx,ax,ge simple ltrgt
• simple ltrgt

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The Shell as a programming language

• variables
• environment
• user created
• positional parameters
• input/output functions
• arithmetic operations
• conditional expressions
• selection structures
• repetition structures

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Variables

• Environment - Special shell variables, keyword
  variables, predefined variables, or standard
  shell variables.
• Examples include TERM, HOME, MAIL, PATH, etc.
• They exist and you can change them but you do not
  initially create them.

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Unix Shell Stuff

• Standard Input and Output
• Standard input - stdin
• Standard output - stdout
• Standard error - stderr
• Redirection
• ls -al gt directory.outltrgt
• ls -al gtgt directory.out ltrgt Appends
• mail gaitrosd lt this.text ltrgt

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Unix Shell Stuff

• Pipes - The output of one command is input to
  another.
• ls -al more ltrgt
• The output of the ls command is given to the
  more command with the pipe () symbol.
• You can also group commands on one line by
  separating them with a semicolon ()

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Unix Shell Stuff

• tee
• A tee allows you to save the output of one
  command while also making it input to another
• ls -al tee output.dat more ltrgt

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Unix Shell Stuff

• Wildcards - A single character that stands for a
  group of characters.
• Let us say that you wanted to list all of the
  files that ended with .out
• ls -al .out ltrgt
• You can put wildcards anywhere
• ls -al fall.out ltrgt
• This gives us all the files that have the word
  fall embedded in it that end in .out.
• The ? is also a wildcard but it only does one
  character at a time. For instance
• ls -al ?ello.out ltrgt
• This lists all the files starting with any
  character and ending in ello.out.

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Unix Shell Stuff

• Special Characters that have special meaning
• \ lt gt ( ) ? ' " / !
• To print these characters you must precede them
  each time with a backslash (\) usually found
  below the pipe() symbol on the keyboard.
• echo "what time is it\?" ltrgt
• It only works on a single character and you must
  put one in front of every special character you
  want to print.
• Backquotes ( also called grave accent marks). The
  backquotes run a unix command inside the shell.

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Unix Shell Stuff
Quote Effiect
\ Cancels the special meaning of a single character inside a string
'string' Cancels the special meaning of any special characters inside the string
"string" Cancels the special meaning of any special characters inside the string except , , and \
string Run any command in the string. Output replaces string. Note these are backquotes.
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Variables

• User Created Variables
• Creating user variables is easy. You just specify
  a name not already in use and assign a value to
  it. Use the set command.
• set stuff gaitrosd/public_htmlltrgt
• To use the value inside the variable you place a
  in front of it.
• ls -al stuff ltrgt
• NOTE typically, user defined variables are in
  lower case to distinguish them from standard
  shell variables.

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Positional Parameters

• Positional parameters are also called read-only.
  They are numbered 1,2,3,.9. and correspond to
  the parameters entered as part of the shell
  command.
• 0 is always the shell script command.
• Each parameter is identified by a space
  separation at the UNIX prompt.
• The is defined in the shell programming
  structure as containing the number of parameters.
  It starts counting with the actual first
  parameter.
• The is the string that contains all the
  arguments.

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Positional Parameters

• Here is an example program called echo.args
• Comment Line
• echo "Welcome to the echo shell program"
• echo "This is an example of positional
  parameters"
• echo "This program can handle up to nine (9)
  parameters"
• echo "You have typed arguments"
• Demonstration of shell script

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Input/Output Functions

• As you have seen, the echo command allows you to
  output items to standard output.
• The read command allows you to input simple items
  into the shell script.
• !/bin/sh
• Illustrates the use of positional parameters,
  user
• defined variables
• and the read command
• echo 'What is your name'
• read name
• echo "Well, name, you typed in arguments"
• echo "They are "
• Demonstration

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Set Command

• Assume we have the program as follows in a file
  called setdate. The set command executes the date
  command and automatically assigns the output to
  the positional parameters.
• set date
• echo "Time 4 5"
• echo "Day 1"
• echo "Date 3 2 6"
• Demonstration

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Arithmetic Operations

• The shell is not intended to do any extensive
  mathematics. You should only perform simple
  integer arithmetic. () () (-) (/) ()
• Use the expr UNIX utility.
• example
• sum expr 1 2 ltrgt
• echo "1 2 is equal to sum"ltrgt
• Note the backquotes in the first line of the
  program.

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Conditional Expressions

• if conditional expression
• then
• commands
• fi
• Most of the time, the conditional expression will
  need the use of the test command.
• Note also commands and the then must start on
  separate lines.
• if conditional expression
• then
• commands
• elif conditional expression
• then
• commands
• else
• commands
• fi

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Conditional Expressions
Argument Test is True if
-d file file is a directory
-f file file is an ordinary file
-r file file is readable
-s file file size is greater than zero
-w file file is writable
-x file file is executable
n1 -eq n2 Integer n1 is equal to n2
n1 -ge n2 Integer n1 is gt n2
n1 -le n2 Integer n1 is lt n2
n1 -ne n2 Integer n1 is not equal to n2
n1 -lt n2 Integer n1 is less then n2
s1 s2 String s1 is equal to string s2
s1 ! s2 String s1 is not equal to string s2
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If statement example - tgif

• set date
• if test 1 Fri
• then
• echo "TGIF"
• elif test 1 Sat test 1 Sun
• then
• echo "Yeah it is the weekend"
• elif test 1 Mon
• then
• echo " Monday stinks "
• elif test 1 Tue
• then
• echo "Thank goodness it is not Monday"
• elif test 1 Wed
• then
• echo "Hump day"
• else echo "Survivor Party Night"
• fi
• echo ""

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The Case Statement

• case word in
• pattern1) command(s)
• pattern2) commands(s)
• . . .
• patternN) command(s)
• esac
• Commands are seperated by semicolongs
• The group of commands is ended by two
  semicolons.
• The OR symbol is just one vertical line ()
• the astrik () marks the default case.

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Case Statement Example

• set date
• case 1 in
• Mon)echo "I do not wake up on Monday"
• Tue) echo "Nothing good on TV might as well
  study"
• Wed) echo "Wednesdays are useless"
• Thu) echo "Watch Alias tape, Love Jenny Garner"
• Fri) echo "Pizza Night"
• Sat) echo "Cartoon day"
• ) echo "It is Sunday"
• echo "I can sleep late and tape Alias"
• esac

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For Loops

• for variable in list
• do
• command(s)
• done
• Example
• for test in "one" "two" "three"
• do
• echo "test"
• done

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While loops

• while condition
• do
• command(s)
• done
• Note condition is the same syntax and behavior
  as in an if statement.

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Until Loops

• until condition
• do
• command(s)
• done
• Note condition is the same syntax and behavior
  as in an if statement.

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Some useful scriptsSource Just Enough Unix by
Paul Anderson. Modified by David Gaitros

• Change a files permissions to
• executable
• File name is chex.
• Pass in the file as a parameter
• if test -f 1
• then
• chmod ux 1
• echo "File 1 is executable"
• ls -l 1
• else
• echo "1 is not a file"
• fi

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Some useful scriptsSource Just Enough Unix by
Paul Anderson. Modified by David Gaitros

• Removing files safely
• File name is DELETE
• set filename 1
• if test ! -f filename
• then echo "Not a valid file"
• else
• echo "Do you really want to delete filename
  (y/n)"
• read choice
• if test choice y test choice Y
• then
• rm filename
• echo "\"filename\"deleted."
• else
• echo "\"filename\" not deleted."
• fi
• fi