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Shell Scripting

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Command Names in Shell Scripting (some examples)? Name Meaning. alias command aliases ... Meaning. everything in quotes is a single string ... Operand Meaning ... – PowerPoint PPT presentation

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Title: Shell Scripting


1
Shell Scripting
  • Lecture 2

2
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.

3
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

4
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

5
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

6
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

7
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

8
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

9
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

10
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

11
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!

12
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

13
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

14
Arithmetic in Shell Scripting
  • Arithmetic expansion
  • ((expression))?
  • echo ((4 2))?
  • 6
  • A 4 B3
  • echo ((4 3))?
  • 12

15
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
16
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

17
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.

18
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

19
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.

20
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

21
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

22
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
23
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

24
Questions?
  • Lab 1 online..
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