CIT 383: Administrative Scripting

CIT 383: Administrative Scripting Slide #1

CIT 383: Administrative Scripting

Commands

Topics

Commands1. System

2. Exec

3. Command Quotes

4. Popen

5. Expect

Time1. UNIX Time

2. Time Zones

3. Calendars

4. Ruby DateTime classes


System

Executes command string in a subshellsystem(“tar cjf ruby.tar.bz2 *.rb”)

system(“cut –d: -f1 /etc/passwd | sort”)

All shell features are availableGlobbing (*/*.c)

Tilde expansion (~jsmith)

I/O redirection

Pipes


System with Multiple Arguments

Multiple arguments have different behaviorFirst argument is name of command.

Later arguments are command line arguments.

None are interpreted by shell.

Examplessystem(“echo *”) prints all files in directory

system(“echo”, “*”) prints a *

system(“tar”, “c”, “f”, “ruby.tar”, “rubyfiles/”)


System Security

Archiving user specified filesfiles = getssystem(“tar cf ruby.tar #{files}”)

What if the user enters “*; rm –rf /”?tar cjf ruby.tar.bz2 *rm –rf /

Use multiple argument form to avoid this bug.files = getssystem(“tar”, “c”, “f”, “ruby.tar”, files)


Exec

Replaces current process by running command.exec(“ls –l”)

# program never reaches this point

Single argument form invokes shellexec(“echo *”)

Multiple argument form does notexec(“echo”, “*”)


Command Quotes

Ruby will run commands in backquotesos = ùname`

os = %x|uname|

Return value is output of command as String.

Command quotes invoke a subshell:files = ècho *`

sortedfiles = ècho * | sort`


Popen

Pipe OpenIO.popen(command_string, mode)

Opens command like a filer: read from command’s STDOUT.

w: write to command’s STDIN.

Similar to command quotes in read mode:uname_fh = IO.popen(‘uname –a’, ‘r’)

unixname = uname_fh.readlines


Popen

Popen offers more control than command quotes.Use less memory (read a line at a time.)Obtain partial output immediately.

Examplesvmfh = popen(“vmstat 5 5”)# Throw away header lines then printvmfh.getsvmfh.getsvmfh.each do |vmline| puts vmlineend


Expect

Automation tool for interactive processes.fsck

ftp

minicom

passwd

telnet

Methodsspawn: start an external command

expect: wait for command to output pattern

send: send string to command as input


ExpectPTY.spawn(‘telnet zork.nku.edu’) do |r_f,w_f,pid|

r_f.expect(/Ûsername.*: /) do w_f.print ‘jsmith’

end r_f.expect('Password:') do

w_f.print passwordend r_f.expect(‘$ ‘) do

w_f.print “passwd #{password} spameggs“end

end


UNIX Time

UNIX time is seconds since the midnight, Jan 1, 1970.

Most OS represent time as 32-bit signed int.Lowest time: 1901-12-13

Highest time: 2038-01-18

OS need to upgrade to 64 bits to avoid Y2038 problem.


Time Zones


UTC: Coordinated Universal Time

Local time at royal observatory at Greenwich– GMT established at 1884 conference.– Prime meridian (0 degrees longitude.)

International Atomic Time (TAI)– Based on atomic clock, established in 1958.– Includes leap seconds to account for lengthening

of day as the Earth’s rotation slows.

Time zones defined by offset from UTC.


CalendarsJulian Calendar

– Introduced by Julius Caesar in 46BC.

– Reform of older Roman calendar system, which had leap months.

– 365 days, 12 months, leap day every 4 years.

– Month Quintilius became July.

Gregorian Calendar– Proposed by Aloysius Lilius.

– Decreed by Pope Gregory XIII in 1582 (adopted by English 1752)

– Dropped 10 days from Julian calendar.

– Changed leap day rules• Leap day every 4 years except• if year is divisible by 100 except• years divisible by 400 are still leap years.


Ruby DateTime classes

DateMutable date objects.

TimeMutable time objects that also include date.

DateTimeImmutable objects with date and time.


Creating a Date object

Date.today– Create Date object for current day.

Date.new(year, month, day)– Create Date object based on year, month, and day.

Date.parse(string)– Create Date object from date stored in string.


Creating a Time object

Time.gm(year <,month,day,hour,min,sec>)

– Create time based on specified values.

– Interpreted as GMT (UTC).

Time.local(year <,month,day,hour,min,sec>)

– Create time based on specified values.

– Interpreted as local set time zone.

Time.new– Create Time object initialized to current time.

Time.now– Same as Time.new.


Creating a DateTime object

DateTime.new– Create Time object initialized to current time.

DateTime.now– Same as Time.new.

DateTime.parse(string)– Create DateTime from date stored in string


Date and Time Arithmetic

Addition and substraction of constants– Increment by day (Date) or second (Time)– Decrement by day (Date) or second (Time)

Subtraction of dates– Determine days between two Dates.– Determine seconds between two Times.


Ranges

Sequence of values1..10?a..?z

Methodsminmaxinclude?(num)

Ranges as Intervals(1..10) === 4 # true(1..10) === 99 # false(1..10) === 2.718 # true


Ranges and Arrays

Ranges provide iterator methods– Including the each method.

Ranges can be converted to Arrays– Use the to_a method.

Ranges are stored efficiently– Only initial and final values are stored.– Do not convert 1..2**32 to an Array.


Date Ranges

today = Date.today

nextweek = today + 7

weekdates = today..nextweek

weekdates.each do |date|

puts date

end


Waiting

sleep(num)– Waits for num seconds.– Does not use any CPU during sleep.

Applications– Wait before retrying an action that failed.– Periodic processes (check file every minute.)

CIT 383: Administrative Scripting Slide #25

References

1. Michael Fitzgerald, Learning Ruby, O’Reilly, 2008.

2. David Flanagan and Yukihiro Matsumoto, The Ruby Programming Language, O’Reilly, 2008.

3. Hal Fulton, The Ruby Way, 2nd edition, Addison-Wesley, 2007.

4. Dave Thomas with Chad Fowler and Andy Hunt, Programming Ruby, 2nd edition, Pragmatic Programmers, 2005.

Documents

CIT 383: Administrative Scripting