Shell Scripting

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Shell Scripting

• Lecture 2

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Intro to Shell Scripting

• What is a shell?
• Process environment accessed by CLI
• Has the ability to start other programs as well
  as a programming language
• What kinds of shells are there?
• BASH Bourne Again Shell
• CSH C Shell
• KSH Korn Shell
• Differences are minor, mostly grammar.

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Components of a Shell Script

• ASCII text file
• Can be used as a script, or in an active shell
• Can call any executable, or other shells
• Language support for things like
• Conditional testing
• Looping
• Variables
• Environment
• Functions
• Arrays

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Running a Shell Script

• First time write of a shell script requires
• change the mode to make executable (preferred
  way)?
• chmod x my.sh
• run from shell executable at command line
• bash my.sh
• based on PATH variable, may need to start
  relative path
• ./my.sh instead of my.sh(may say no such
  file)?
• Executing shell in debug mode
• bash -x my.sh

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Meta Characters in the Shell

• Pattern Matches
• Every file in the current directory
• ? Files consisting of one character
• ?? Files consisting of two characters
• ?? Files consisting of two or more
  characters
• abcdefg Files consisting of a single letter
  from a to g.
• gfedcba Same as above
• a-g Same as above
• a-cd-g Same as above
• a-zA-Z0-9 Files that consist of a single
  letter or number
• !a-zA-Z0-9 Files that consist of a single
  character not a letter or number
• a-zA-Z Files that start with a letter
• ?a-zA-Z Files whose second character matches
  a letter.
• 0-9 Files that end with a number
• ?0-9 Two character filename that end with a
  number
• .0-9 Files that end with a dot and a
  number

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Special Characters in Shell Scripting

• Character Meaning
• pipe for I/O
• end of cmd - background
• terminate statement
• lt gt less than greater than
• ( ) cond testing
• variable
• " ' quotes
• \ remove meaning for next char
• ltspacegt lttabgt ltnewlinegt space tab newline
• wildcard (be careful!) example
• ? decimal exit status
• cond testing
• comment
• used for home directory

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Reserved words in Shell Scripting

• Word Meaning
• ! logical NOT
• braces
• case esac case open/close
• do done do loop syntax
• if then else elif fi conditional testing syntax
• for in for loop syntax
• until until loop syntax
• while while loop syntax

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Command Names in Shell Scripting (some
examples)?

• Name Meaning
• alias command aliases
• true / false logical true/false values
• jobs foreground/background
• wait wait until cmd executes
• bg / fg force a job in background or
  foreground
• cd change directories
• kill send process signal
• pwd print working directory
• fc fix command

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Quoting in Shell Scripts

• 3 types of quotation, very different meanings
• double quote
• Meaning
• everything in quotes is a single string
• evaluate and plug in any variables
• single quote '
• Meaning
• everything in quotes is a single string
• do NOT evaluate variables, literal on everything
• back quotes
• Meaning
• execute string in quotes

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Quoting example in Shell Scripting

• VARABC
• Double Quotes
• echo Hello World - VAR
• Hello World - ABC
• Single Quotes
• echo Hello World - VAR
• Hello World - VAR
• Back Quotes
• echo VAR
• ERROR can not execute command ABC

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Variables in Shell Scripting

• namevalue pairs
• A2
• use as A from then on..
• Name can be the following
• starts with a value a-Z, not numeric
• any string that does not contain a special
  character
• Value can be anything, be careful of special
  characters!!
• Environmental variables already set
• export!

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I/O and shell scripting

• I/O is input, output and error
• Known to the shell as
• Input comes from the 'lt' character
• cat lt filename
• Output uses the 'gt' character
• cat filename gt other-filename
• Error uses the '2gt' characters
• 2 stands for the file descriptor 2, output is 1
  and input is 0
• cat filename gt other-filename 2gt errors
• Pipe '' is the standard out of cmd1 as standard
  in of cmd2
• cmd1 cmd2

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I/O and Shell Scripting con't

• Appending to an existing file
• Use the gtgt characters to append
• cat filename gtgt filename2
• note if filename2 does not exist, it will
  create and add text
• Copying where output goes
• Use the '' character
• cat filename gt filename2 2gt1
• Here-to document
• Ability to run commands until the word is met
• cat ltlt EOF
• do something
• EOF

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Arithmetic in Shell Scripting

• Arithmetic expansion
• ((expression))?
• echo ((4 2))?
• 6
• A 4 B3
• echo ((4 3))?
• 12

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Conditional Testing

• if then elif else fi
• based on true false values
• if test then
• do something
• fi
• if A -eq 1 then
• echo 'A' is A
• fi
• if ls -la then
• echo Can ls -la
• fi

if A -ne 1 then echo A is not 1 elif
A -gt 2 then echo A greater than
2 else echo A less than 1 fi
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Conditional Testing what you can test for?

• if test is true do something
• Can test for the following
• exit value for executable
• e.g. if ls -la then
• using the test syntax
• e.g. a ABC - string test
• !
• e.g. a -eq 12 - numeric test
• -eq, -lt, -gt, -ne, -le, -ge

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Conditional Testing - cont

• Operand Meaning
• If the right hand side matches a pattern,
  (i.e., similar to filename matching, with
  asterisks and question marks.) the condition is
  true.
• ! If the right hand side doesn't match a
  pattern, the condition is true.
• -d var True if the file is a directory.
• -e var True if the file exists.
• -f var True if the file is a file. (I.e., not
  a directory)
• -o var True if the file is owned by the user.
• -r var True if the user has read access.
• -w var True if the user has write access.
• -x var True if the user has execute access.
• -z var True if the file is zero-length.

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Conditional testing con't case

• case statements
• case "year" in
• 0-90-9)?
• year19year
• yearsexpr year - 1901
• 0-90-90-90-9)?
• yearsexpr year - 1901
• )?
• echo 1gt2 Year \"year\" out of range
  ...
• exit 127
• esac

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Looping constructs

• 3 different kinds of looping
• for
• while
• until
• All accomplish the following, but 'for' is
  different because it iterates on the variable/cmd
  sent to the command.

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Looping constructs - for loop

• For each item in command
• for VARIABLE in LIST
• do
• something
• done
• alphabet"a b c d e" Initialise a string
• count0 Initialise a counter
• for letter in alphabet Set up a loop control
• do Begin the loop
• countexpr count 1 Increment the
  counter
• echo "Letter count is letter" Display
  the result
• done End of loop

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Looping constructs - while loop

• while test is true do something
• while TRUE
• do
• something
• done
• alphabet"a b c d e" Initialise a string
• count0 Initialise a counter
• while count -lt 5 Set up a loop
  control
• do Begin the loop
• countexpr count 1 Increment the
  counter
• positionbc count count - 1
  Position of next letter
• letterecho "alphabet" cut
  -cposition-position Get next letter
• echo "Letter count is letter"
  Display the result
• done End of loop

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Looping constructs - until loop

• until test is true do something
• until TRUE
• do
• something
• done

alphabet"a b c d e" Initialise a
string count0 Initialise a
counter while count -lt 5 Set up a
loop control do Begin the loop
countexpr count 1 Increment the
counter positionbc count count - 1
Position of next letter letterecho
"alphabet" cut -cposition-position Get
next letter echo "Letter count is
letter" Display the result done
End of loop
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Functions

• Shells have the ability to utilize function calls
• deal with a file, add people one at a time
• do_file()?
• while parse_one
• etc...
• etc...
• take standard input (or a specified file)
  and do it.
• if "1" ! "" then
• cat 1 do_file
• else
• do_file
• fi

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Questions?

• Lab 1 online..