Shell Programming

1
Shell Programming

• Kaustav Ghoshal
• kghoshal_at_tamu.edu

2
The Basics

• What is a shell ?
• Why use a shell script?
• Creating a Script
• Replace find . -name file print by a command
  sfind file
• Create a shell script
• vi sfind
• cat sfind
• find . -name 1 -print
• chmod x sfind
• cd /sbin
• sfind init
• ./sfind /etc/password

3
The Basics (contd..)

• The use of the !
• Comments
• Argument Checking
• if -ne 3 then
• echo 1gt2 Usage 0 19 Oct 91
• exit 127
• fi
• This script requires three arguments and works
  accordingly.

4
Exit Status

• Exit status
• is the year out of range for me?
• if year -lt 1901 -o year -gt 2099 then
• echo 1gt2 Year \"year\" out of range
• exit 127
• fi
• etc...
• All done, exit ok
• exit 0
• A non-zero exit status indicates an error
  condition of some sort while a zero exit status
  indicates things worked as expected.

5
Exit Status

• Using exit status
• Exit codes are important for those who use your
  code. Many constructs test on the exit status of
  a command.
• The conditional construct is
• if command then
• command
• fi
• For example,
• if tty -s then
• echo Enter text end with \D
• fi

6
File Descriptors

• Stdin, Stdout, Stderr
• Standard input, output, and error are file
  descriptors 0, 1, and 2. Each has a particular
  role and should be
• used accordingly
• is the year out of range for me?
• if year -lt 1901 -o year -gt 2099 then
• echo 1gt2 Year \"year\" out of my range
• exit 127
• fi
• etc...
• ok, you have the number of days since Jan 1,
  ...

7
File Descriptors

• case expr days 7 in
• 0) echo Mon
• 1) echo Tue
• etc...
• Error messages should appear on stderr not on
  stdout! Output should appear on stdout.

8
The quotes

• Quotes
• Single Quotes
• The result is one word
• grep gid /etc/group awk -F print 1
• Double Quotes
• Within double quotes you have variable
  subsitution.
• if ! "parent" then
• parentpeople/group/user
• fi
• Back Quotes
• Back quotes mean run the command and substitute
  the output.
• TODAY(set \date\ echo 1)

9
Language Constructs

• Language Constructs
• For loop iteration
• Substitute values for variable and perform task
• for variable in word ...
• do
• command
• done
• For example
• for i in cat LOGS
• do
• mv i i.TODAY
• cp /dev/null i
• chmod 664 i
• done
• Alternatively you may see
• for variable in word ... do command done

10
Language Constructs

• Case
• Switch to statements depending on pattern match
• case word in
• pattern pattern ... )
• command ...
• esac

11
Language Constructs

• For example
• 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

12
Language Constructs

• Pattern Matching in case Statements
• ?) Matches a string with exactly one character
  like a, 3,!
• ?) Matches a string with one or more characters
  (a non-empty string).
• yYyYeEsS)
• Matches y, Y or yes, YES, YeS, etc. The means
  "or."
• //0-9)
• Matches a file pathname, like /xxx/yyy/somedir/fil
  e2, that starts with a slash, contains at least
  one more slash, and ends with a digit.
• 'What now?')
• Matches the pattern What now?
• "msgs")
• Matches the contents of the msgs variable

13
Language Constructs

• Conditional Execution
• Test exit status of command and branch
• if command
• then
• command
• else
• command
• fi
• For example
• if -ne 3 then
• echo 1gt2 Usage 0 19 Oct 91
• exit 127
• fi
• Alternatively you may see
• if command then command else command fi

14
Language Constructs

• While/Until Iteration
• Repeat task while command returns good exit
  status.
• while until command
• do
• command
• done
• For example
• for each argument mentioned, purge that
  directory
• while -ge 1 do
• rm 1
• shift
• done
• Alternatively you may see
• while command do command done

15
Variables

• Variables
• Variables are sequences of letters, digits, or
  underscores beginning with a letter or
  underscore. To get the contents of a variable you
  must prepend the name with a .
• Numeric variables (eg. like 1, etc.) are
  positional variables for argument communication.
• Variable Assignment
• Assign a value to a variable by variablevalue.
  For example
• PATH/usr/ucb/usr/bin/bin export PATH
• Or TODAY(set \date\ echo 1)

16
variables

• Referencing Variables
• Use variable (or, if necessary, variable) to
  reference the value.
• if "USER" ! "root" then
• PATHHOME/binPATH
• else
• PATH/etc/usr/etcPATH
• fi
• The braces are required for concatenation
  constructs.
• p_01 ?The value of the variable "p_01".
• p_01 ?The value of the variable "p" with "_01"
  pasted onto the end.

17
Export

• Exporting Variables
• Variables are not exported to children unless
  explicitly marked.
• We MUST have a DISPLAY environment variable
• if "DISPLAY" "" then
• if tty -s then
• echo "DISPLAY (hostname0.0)? \c"
• read DISPLAY
• fi
• if "DISPLAY" "" then
• DISPLAYhostname0.0
• fi
• export DISPLAY
• fi

18
Variable reference

• Conditional Reference
• Operator Explanation
• var-default If var is not set or is empty,
  use default instead.
• vardefault If var is not set or is empty,
  set it to default and use that value.
• varinstead If var is set and is not empty,
  use instead. Otherwise, use nothing (null
  string).
• var?message If var is set and is not empty,
  use its value. Otherwise, print message, if any,
  and exit from the shell. If message is missing,
  print a default message (which depends on your
  shell).

19
Variable reference

• Conditional Reference (example)
• !/bin/sh
• if cp "1" "1.bak"
• then VISUAL-/usr/ucb/vi "1
• exit
• USE STATUS FROM EDITOR
• else echo "basename 0 quitting can't make
  backup?" 1gt2
• exit 1
• fi

20
Speacial variables

• Special Variables
• Current process id. This is very useful for
  constructing temporary files.
• tmp/tmp/cal0
• trap "rm -f tmp /tmp/cal1 /tmp/cal2"
• trap exit 1 2 13 15
• /usr/lib/calprog gttmp
• ?
• The exit status of the last command.
• command
• Run target file if no errors and ...
• if ? -eq 0
• then
• etc...
• fi

21
functions

• Functions
• Syntax is
• name ()
• commands
• For example
• purge()
• if ! -d 1 then
• echo 1 No such directory 1gt2
• return
• fi
• Within a function the positional parmeters 0,
  1, etc. are the arguments to the function (not
  the arguments to the script).
• Within a function use return instead of exit.

22
More useful stuff

• Test
• if test expression then
• etc...
• if expression then
• etc...
• Useful expressions are
• test -w, -r, -x, -d, ... filename
• test n1 -eq, -ne, -gt, ... n2
• test s1 , ! s2
• test cond1 -o, -a cond2
• Binary or binary and use ! for unary negation.
• For example
• if year -lt 1901 -o year -gt 2099 then
• echo 1gt2 Year \"year\" out of range
• fi

23
String matching

• String matching
• while -ge 1 do
• case 1 in
• -c) rateecho 1 cut -c3-
• -c) shift rate1
• -p) prefixecho 1 cut -c3-
• -p) shift prefix1
• -) echo Usage exit 1
• ) disks break
• esac
• shift
• done

24
Traps

• Trapping Exits Caused by Interrupts
• stat1
• temp/tmp/zpg
• trap 'rm -f temp exit stat' 0
• trap 'echo "basename 0Quitting early." 1gt2'
  1 2 15
• Signals
• 0 EXIT exit command
• 1 HUP When session disconnected
• 2 INT Interrupt - often CTRL-c
• 3 QUIT Quit - often CTRL-\
• 15 TERM From kill command

25
Miscellaneous Stuff

• Reading from the Keyboard
• echo -n "Enter first and last name " read fn ln
  
• The character
• while do commands done
• if something then else commands fi

26
Mathematical expressions

• Expr
• Syntax expr arg1 operator arg2 operator arg3
  ...
• Arithmetic Operators like - /
• Relational Operators like ! gt gt lt lt
• Logical operators like