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CTEC 1863 – Operating Systems

CTEC 1863 – Operating Systems. Shell Scripting. Overview. How shell works Command line parameters Shift command Variables Including predefined/specials vars Functions. Shells. sh is a command interpreter It supports a powerful command language

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CTEC 1863 – Operating Systems

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  1. CTEC 1863 – Operating Systems Shell Scripting

  2. Overview • How shell works • Command line parameters • Shift command • Variables • Including predefined/specials vars • Functions CTEC1863 - 2009F

  3. Shells • sh is a command interpreter • It supports a powerful command language • Each invocation of the sh program is called a shell CTEC1863 - 2009F

  4. Shells (2) • OS’s allow invoking commands one at a time from a terminal • Shells allow users to: • Combine commands to form new commands • Pass positional parameters to a command • Add or rename commands • Execute commands within loops • Execute commands conditionally • Send commands into the background CTEC1863 - 2009F

  5. How it works: CTEC1863 - 2009F

  6. How it works: • When you type a command: • Your shell makes a copy of itself through the fork system call (PID 230) • Your shell puts itself to sleep (via the wait system call) until its child process (PID 230) calls exit • The child process calls exec(date) to overwrite itself with the code for date • When the date command is finished, it calls exit() • The child process (230) dies, and your shell wakes up again, and displays the prompt CTEC1863 - 2009F

  7. Variables • Spring into existence by being mentioned • May be assigned string values • Syntax: NAME=STRING • Thereafter "$NAME" will yield "STRING" • No spaces around the = sign!!!!! • Double quotes around the right-hand side allow STRING to contain semicolons, spaces, tabs, and newlines CTEC1863 - 2009F

  8. Examples: $ echo NAME NAME $ NAME=Howard $ echo $NAME Howard $ HIS="/usr/user3" $ ls $HIS CTEC1863 - 2009F

  9. Positional Parameters • Implicitly created when a shell script runs • These variables are known as: $0, $1, ..., $9 Try this script: echo "Example of positional parameters" echo "$5" echo $1 echo $2 echo $3 $4 $ PARAMS These parameters are passed Also try changing “$5” to ‘$5’ CTEC1863 - 2009F

  10. shift • Used when more than 10 ($0-$9) parameters • After the shift command is executed, the positional parameters from $2 ... are renamed $1 ... • E.g. if commandfile is invoked as: commandfile arg1 arg2 arg3 arg4 arg5 arg6 $0 $1 $2 $3 $4 $5 $6 • After a shift command is executed: commandfile arg1 arg2 arg3 arg4 arg5 arg6 $0 gone $1 $2 $3 $4 $5 CTEC1863 - 2009F

  11. Shift example Try this script: echo "Example of shift command" echo $1 shift echo $1 shift echo $1 With: $ ./shiftit shows shift command CTEC1863 - 2009F

  12. set command • Forces values into the positional parameters • For example: set -- abc def ghi is equivalent to: $1=abc $2=def $3=ghi • Note: • Positional parameters are not allowed on the left hand side of an assignment statement. Try this script: set -- THESE THREE VALUES echo $1 $2 $3 CTEC1863 - 2009F

  13. Shell Maintained Variables • HOME: Upon login, the shell assigns a user's login directory • IFS: Internal field separators, default is blank, tab, newline • MAIL: Pathname of the file where your mail is deposited • PATH: Search path used to find commands CTEC1863 - 2009F

  14. Special Variables $# $? $! $$ $- $# records the number of arguments passed to a shell procedure e.g. $ someproc x y z sets $# to 3 • Tell if correct number of args passed in CTEC1863 - 2009F

  15. Special Variables (2) $? contains the exit status of the last command executed • Zero indicates successful completion • Anything from 1 to 255 indicates failure • Often only want to execute next command if last was successful $! contains the process number of the last process run in the background (using &) CTEC1863 - 2009F

  16. Special Variables (3) • $$ contains the process number of the currently running process. • Process numbers are unique among all existing processes. • Often used if a shell procedure needs to create temporary files: … temp=$HOME/temp.$$ ls > $temp … rm $temp … CTEC1863 - 2009F

  17. Special Variables (4) • $- is string containing the names of all the flags currently turned on in the shell • Also… • $* is not considered a special variable • Contains $1, $2, ... separated by spaces CTEC1863 - 2009F

  18. Testing Variables • Test command • Usage: test EXPRESSION • test evaluates EXPRESSION to true or false • True  exit with STATUS 0 • False  exit with STATUS !0 • test is used for three categories of testing: • Files • Strings • Numbers CTEC1863 - 2009F

  19. Functions • Sequence of commands • Defining: function_name () { statements } • Use: function_name CTEC1863 - 2009F

  20. Functions (2) • Arguments: print_args() { echo “Arg 1 is $1” echo “Arg 2 is $2” } • Passing arguments: print_args arg1 arg2 CTEC1863 - 2009F

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