Operating Systems Lecture 2 UNIX and Shell Scripts

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Operating SystemsLecture 2UNIX and Shell
Scripts
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C Programs with Arguments
A C(C) program may be called with arguments
(like parameters for the whole program. Arguments
may be used for 1. Options used for decisions
in main( ) 2. Strings that change in output
statements 3. Filenames for input and output
files 4. Strings can be changed to values with
atoi( ) Arguments have built-in names argc
Argument count (the number of arguments
given) argv List of arguments (array of
strings)
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Calling a program with Arguments
Example gt myProgram inputFile outputFile argc
3 (myProgram counts as one of the
arguments) argv holds argv0
"myProgram" argv1 "inputFile" argv2
"outputFile"
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C Code using argc and argv
include ltiostream.hgt include ltfstream.hgt int
main(int argc, char argv ) ifstream
inFile ofstream outFile if (argc !
3) //Error checking cout ltlt "Usage myProgram
inFile outFile " ltlt endl else
inFile.open(argv1) if (!inFile) //Error
checking cout ltlt argv1 ltlt " not opened." ltlt
endl outFile.open(argv2) if (!outFile)
//Error checking cout ltlt argv2 ltlt " not
opened." ltlt endl //Rest of code for input and
output from files... return 0
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Using atoi
All arguments are input as strings. If you want
to use numeric values, you must convert from the
string to a number include ltstdlib.hgt ... int
number, square ... number
atoi(argv1) square number number ...
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UNIX Commands
Commands you should already know ls, pwd, cd,
mkdir, rmdir cp, mv, rm substitution
character Commands you should learn echo, cat,
more, grep pipe that connects stdout with
stdin e.g. ls -l grep Aug Use man to find
out about commands. A good UNIX tutorial can be
found here http//www.ee.surrey.ac.uk/Teaching/Un
ix/index.html
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Kernel vs. Utilities
The kernel is a process that is always
executing. A process is a program that is loaded
into memory and executing Utilitites reside on
the disk (e.g. grep, lpr, etc.) The shell
program is a utility. The shell knows some
built-in commands that don't have to be read off
the disk. (cd, pwd, etc)
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Shells
The shell is a command line interpreter. Its
function is to get and execute the next
statement. Common shells Bourne shell
(/bin/sh) Korn shell (/bin/ksh) C
shell (/bin/csh) T shell (/bin/tcsh) GNU
Bourne-Again Shell (/bin/bash) Z shell
(/bin/zsh) The Bourne shell is standard. The
shell is just a program. Anyone can write their
own custom taylored shell.
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The Shell Environment
The shell environment is a list of associated
strings in the shell PATH path or paths used
to look for programs or utilities. e.g.
/opt/local/bin HOME Location of home
directory e.g. /fac/ycjiang SHELL Current
shell e.g. /bin/csh Type setenv to see a list
of current environment variables. Use setenv to
change environment setenv TERM xterm Change
terminal type to xterm
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Unix shell

• shell is
• simply a macro processor that executes commands
• a command interpreter Which provides the user
  interface to the rich set of GNU utilities
• a programming language Allow these utilities to
  be combined.
• Shell scripts are
• Files containing commands can be created, and
  become commands themselves.
• These new commands have the same status as system
  commands in directories such as '/bin', allowing
  users or groups to establish custom
  environments.
• Ex.
• !/bin/bash saved as fred
• ls grep .sxw /home/ycjianggt ./fred
  --------- unix1.sxw unix2.sxw ...
• A shell allows execution of GNU commands,
• both synchronously and asynchronously.
• The shell waits for commands given synchronously
  to complete before accepting more input
• Commands given asynchronously continue to execute
  in parallel with the shell while it reads and
  executes additional commands.

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More intro on shell

• Shells may be used interactively or
  non-interactively
• they accept input typed from the keyboard or from
  a file.
• Shells also provide a small set of built-in
  commands (builtins)
• implementing functionality impossible or
  inconvenient to obtain via separate utilities.
• eg cd, break, continue, and exec
• cannot be implemented outside of the shell
  because they directly manipulate the shell
  itself.
• The history, getopts, kill, or pwd builtins,
  among others, could be implemented in separate
  utilities, but they are more convenient to use as
  builtin commands.
• While executing commands is essential, most of
  the power (and complexity) of shells is due to
  their embedded programming languages.
• Like any high-level language, the shell provides
  variables, flow control constructs, quoting
  (removing special meaning from a sequence of
  characters see bash ref), and functions.
• Shells offer features geared specifically for
  interactive use.
• These interactive features include job control,
  command line editing, history and aliases.

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Examples

• http//www.gnu.org/software/bash/manual/bashref.ht
  ml

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UNIX Security

• Each user has
• userid, password, home directory (can login many
  times simultaneously)
• Each user's files and dirs from tree rooted in
  home dir
• Usually want your files and dirs to be able to be
  listed, displayed and modified only by you
• Possible to give other users permissions to
  access your files - in various ways
• Owners and Groups
• Each file and dir has - an owner and a group
  associated with it Owner
• When you create a file, you become its owner
  (usually) Group
• Users can join "groups" of other users with whom
  they can share files dirs
• Users can join many groups, but have one primary
  group
• Users can "work" in different groups
• - to change groups newgrp newgroup sysadmin is
  only person who can create groups and add you to
  groups
• When user creates file
• - file's group is group the user was in when the
  file was created
• - typically your primary group
• chgrp groupname filename
• - file's group can be changed can only be changed
  by a current group member - owner must be in
  group groupname
• - after change, only members of new group have
  power over file
• chown userid filename

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File Permissions

• Files have permissions for the user/owner, group
  and others.
• You can view the permissions by typing ls -l
• -rwxr--r-- 1 ycjiang fac 81 Jan 3 2153 myFile
• The information given is
• file type (- plain file)
• permissions for user (rwx), group (r--) and
  other (r--)
• number of hard links (1)
• user/owner name (ycjiang)
• group name (fac)
• size of file (81)
• date and time last modified (Jan 3 2153)
• filename (myFile)

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File Permissions

• Permissions on a file dictate how owner, group
  members and other users may operate on files
• - read, write, and execute permissions for each
  file, dir
• - each of user (u), group members (g), and other
  users (o) have a distinct set of read (r),
  write (w), and execute (x) permissions on a
  file/dir Numeric equivalents
• u g o
• rwx rwx rwx
• 111 111 111
• 7 7 7
• 101 011 100
• 5 3 4
• chmod 534 dum
• r file file can be read and copied
• dir files names in directory can be listed
  (cannot see file contents --need r on files and x
  on dir)
• w file file can be modified or deleted
• dir file names can be added/deleted to/from
  dir (can modify files without w on dir)
• x file file can be executed if "executable"
  (binary, script)
• dir If name known can read file if file is
  r, or see into subdirectory dir but can't list
  contents of dir - to see known file's contents,
  need r on file, x on dir

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To rephrase

• r file can be read, copied (see file
  contents)
• w file can be modified, deleted
• x file can be executed (if "executable" binary,
  script)
• For directories
• r can list files in directory (r alone cannot
  read file or get permissions)
• w add/delete files from directory
• x allows reading IF YOU KNOW FILENAME can't
  list files
• Superuser has ALL permissions
• Some special bits (UNIX for the Impatient Pg
  47)
• set-uid bit s replacing x in user (owner's)
  permissions
• - set user id (set-uid) bit on your program
  file
• - "other" runs program which accesses a file
• - file now accessed with permissions of owner,
  not "other normally, when "other" runs a program
  and it accesses a file, the file permissions seen
  are "other" permissions on that file
• - allows others to access a file you own but only
  through YOUR program and thus in limited ways
  like private data and a get-method in OO
• set-gid bit s replacing x in group
  permissions
• - set group id (set-gid) bit on your program
  file
• - "other" runs program which accesses a file
• - file now accessed with permissions of group,
  not "other
• sticky bit t replacing x in others permissions

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To rephrase (Contd)

• To check this (see chmod just below)
• - in home directory create subdirectory test
• - in test create file named dum (contains abc
  def) To check this (see chmod just below)
• - cd test
• - chmod 700 dum user has read, write ,
  execute
• - cd .. - pretend with chmod 000 test
• - try ls ./test gt Permission denied
• - try cat ./test/dum gt Permission denied change
  to chmod 100 test user x--
• - try ls ./test gt Permission denied
• - can see permissions ls -al
• - try cat ./test/dum gt abc def
• To allow anyone to execute file
  /home/smith/labs/myfile
• - need x perms on myfile,
• - need x perms on dirs down path to
  /home/smith/labs so others can get at the file in
  the tree.

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Web Accessibility

• To make file.html in public_html "web accessible"
  from a link on your page (file name "known")
• - need x on public_html and x on its parent (your
  home)
• - need r on file.html
• - don't provide any more access than that !!
• Can see perms on files with -l option of ls
• -rwxr-xr-- 1 ycjiang nobody 1734 Dec 21 2002
  lab1
• user has read,write and execute perms on
  lab1, group has only read and execute perms on
  lab1, all others have only read perms on lab1
• Changing permissions
• - owner (only) can change permissions on a
  file/directory chmod command
• chmod (ugoa)(-)(rwx) file/dir name(s) changes
  to exactly those given perms /- add, remove
  given perms
• e.g., chmod gw lab1 -rwxrwxr--
• chmod ug-x lab1 -rw-rw-r--
• chmod arwx lab1 -rwxrwxrwx chmod gorx lab1
  -rwxr-xr-x
• chmod gx,ow lab1 -rwxxrwx

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Note

• - - -(000) 0
• - - x(001) 1
• - w -(010) 2
• - w x (011) 3
• r - -(100) 4
• r x (101) 5
• r w - (110) 6
• r wx (111) 7
• chmod 160 lab1 sets only x for user, only rw for
  group and no perms for others
• COMMON! Default permissions umask sets up default
  permissions for all subsequently created
  files/dirs tells which perms to EXCLUDE
• e.g., umask 022 exclude write perms for group
  and other don't exclude any perms from user
  (excluding from what was set up as default by
  sysadmin--so typically rw for user)

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Changing Permissions
File types - plain file, d
directory Permissions r read permission, w
write permission, x execute permission. Use
chmod to change permission for user, group, other
or all chmod ar filename everyone gets read
permission chmod gx filename group gets
execute permission chmod uw filename user gets
write permission chmod o-w filename others lose
write permission chmod og-rw filename group and
others lose read and write permission
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Exercise

• If you had a directory called dog that
• had read permissions for everyone, and write
  permissions only for the owner, and execute
  permissions for the group and others,
• what would the permissions part of a "long
  listing" of the directory look like?
• Assuming you were in the parent directory of dog,
  what is the command that would remove all
  permissions for group and other?
• what command would give execute permissions to
  everyone, read permissions to the group, and
  remove write permissions for the user?
• You have default permissions for all files you
  create.
• create 3 files, called tst1, tst2 and tst3, with
  vim (just enter one word or sentence in each
  file).
• Then from your home directory, use whatever
  commands you need to discover what the default
  permissions are on all files you create

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Shell Programming

• Shell scripts must be marked as executable
• chmod ax myScript
• 2. Use to start a comment.
• Comments run from to the end of the line.
• 3. All shell scripts begin with the interpreter
  you want to use
• !/bin/sh
• Example
• !/bin/sh
• who grep ycjiang
• exit 0

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Running a shell script

• To run a UNIX script
• Type the script into a file.
• Change the file permission to executable.
• Execute it (by typing the filename at the
  prompt).

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Shell Variables
Shell variables are stored as strings Example
!/bin/sh x1 Note No spaces in
assignment. If space after x, thinks x is a
command echo The value of x is x x prints
the value of variable x echo The home directory
is HOME echo The current shell is SHELL (Note
to debug, use -x sh -x scriptFileName This
will list the commands as they are executed.)
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Using Quotes
Single quote Groups together characters until
end quote. is not processed. Example
!/bin/sh grep Constance ycjiang
/etc/passwd Tries to open ycjiang as a file
grep 'Constance ycjiang' /etc/passwd Search
es for Constance ycjiang in passwd file
x1 echo x echos 1 echo 'x' echos
x exit 0
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Double Quotes
Double quotes act like single quotes, except the
is processed !/bin/sh x1 echo x echos
the value of x echo "x" echos the value of
x address"College of the Holy Cross" echo
address echos College of the Holy Cross echo
"address" ditto exit 0
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More Quotes
Backslash (\) Places a single quote around a
character \gt is the same as 'gt' Back quote
() Tells shell to execute the enclosed command
and insert the output here !/bin/sh echo
There are who wc -l users logged on exit
0 Try these examples out for yourself!
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Devices

• device
• usually a piece of equipment for storing or
  communicating data, e.g., printer, disk drive,
  terminal, modem
• In UNIX, we access devices as if they were
  (special) files, typically in dir /dev e.g., a
  printer might be "file" /dev/lp1
• - therefore cp myfile /dev/lp1 - prints myfile
  on line printer
• - echo "fred" gt /dev/usb/lp0 prints on my
  printer (HPLJ3015) as superuser - set to convert
  text to postscript stdin/stdout/stderr are
  /dev/stdin, /dev/stdout, /dev/stderr
• - echoing a line ONTO stderr could be done
• echo blah blah gt /dev/stderr - use stderr
• diff blah blat 2gt errorfile since 0,1,2
  stdin/out/err
• diff blah blat 1gt errorfile creates empty
  errorfile stderr goes to screen

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Devices Contd

• When OS recognizes an operation on a special
  file, it calls a pgm called a device driver to do
  the op (e.g., cause data to be displayed on
  screen)
• character devices transfers info char by char
  (printer, keyboard, modem)
• block devices transf. info in batches of chars,
  called blocks (disk, tape)
• terminals special char. devices, since chars are
  interpreted - tabs transformed into blanks -
  don't see every char you ever typed -e.g.
  those just before a "backspace" char
• Null Device (/dev/null) the garbage can, black
  hole -- data can go in but never come back out. -
  sending output to /dev/null throwing it away
• - sometimes commands produce output we want to
  disregard When OS recognizes an operation on a
  special file, it calls a pgm called a device
  driver to do the op (e.g., cause data to be
  displayed on screen)
• terminals special char. devices, since chars are
  interpreted - tabs transformed into blanks -
  don't see every char you ever typed -e.g.
  those just before a "backspace" char

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Exercise

• Create a file called tst1, and a directory called
  dog, in your home directory
• Copy tst1 to a file called cat in directory dog.
  Move into dog.
• Remove write and execute permissions for user
  (that's you!) on file cat.
• Now try to erase cat. What happens? Why?

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(I/O) Streams

• UNIX commands, and pgms, do I/O
• e.g., ls writes results to screen (output)
• passwd reads your old and new password from the
  keyboard
• How? The shell associates streams with the
  command.
• - stream like a tunnel (output sent down, input
  received)
• The shell assigns 3 standard streams to any
  command stdin, stdout, stderr
• When command needs to read input, it looks in
  stdin, and reads whatever is there When command
  produces output, it stdout stderr
  
  
• The shell normally attaches other end of the
  streams to devices, such as the keyboard and
  monitor (screen) sends it down stdout
• When command produces error message, it sends it
  down stderr.
• e.g., passwd ----- stdin
• e.g., -keyboard passwd monitor
• -- stdin stdout
  stderr monitor
• When passwd wants input (e.g., the current
  password) reads from stdin reading from
  keyboard (passwd doesn't know or care that it is
  the keyboard, just reads from stdin)
• Output similar- when passwd encounters error,
  writes error msg on stderr (it ends up appearing
  on monitor, but passwd doesn't know or care)

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Redirecting I/O

• Normally, the shell will use standard input and
  standard output for executing commands.
• You can redirect the standard input and output
  using lt and gt
• ls -l gt filex
• Redirect the output of ls -l into the file named
  filex.
• Using gtgt allows you to append output to a file
• ls -l gtgt filex
• Append the output of ls -l to the end of file
  named filex.
• The redirection constructs permit fine-grained
  control of the input and output of those
  commands.

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I/O Redirection Examples

• for command cat myfile
• - shell attaches other end of stdin to the file
  myfile read from stdin reading characters from
  file myfile- the shell redirected stdin to come
  from a file (stdout and stderr to monitor still)
• You can tell shell to redirect std streams ls gt
  myfile - shell attaches other end of stdout to
  file myfile
• - to see output of ls command - must cat, edit,
  more, less, etc.
• myfile gt myfile clobbers myfile appends Redirect
  input similarly
• e.g., if cat cmd is given no argument, stdin
  is keyboard
• /home/ycjianggt cat gt outfile my first line
  my second line d lt end of file in UNIX (z
  in Win) now file outfile contains my first
  line my second line
• /home/ycjianggt cat outfile gt my first line my
  second line...
• /home/ycjianggt cat lt outfile (same effect)file
  outfile displayed on screen
• /home/ycjianggt cat ltinfile gtoutfile
• /home/ycjianggt lt nothing displayedTo redirect
  stderr use "2gt" (review!)e.g.,
  /home/ycjianggt ls labs 2gt errfile
  error msgs written to file errfile instead of
  screen (if, for example, file labs did not
  exist...)
• Use redirect and /dev/null to throw output away
  (review!)
• e.g., /home/ycjianggt ls nonExistantFile
  /bin/ls nonExistantFile No such file or
  directory and errors printed on stderr. but,
• /home/ycjianggt ls nonExistantFile 2gt/dev/null
• - each stream given a number by shell - stdin 0,
  stdout 1, stderr 2 (ls gtoutfile ls 1gtoutfile)