Transcript UNIX Admin Tools

UNIX Admin Tools
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Overview
• Review of file manipulation utilities
• UNIX process subsystem
• Overview of the UNIX shells csh/ksh
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File Attributes
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Stored in the file I-node
File’s ownership: user and group
file permissions: read, write, execute
file modification times
file type: regular, directory, link, symbolic
link, special file
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Utilities for Manipulating file
attributes
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chmod
change file permissions
chown
change file owner
chgrp
change file group
only owner or super-user can change file
attributes
• upon creation, default permissions given to
file modified by process umask value
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File Permissions
• Three types of permissions:
• read, process may read contents of file
• write, process may write contents of file
• execute, process may execute file
• three sets of permisions:
• permissions for owner
• permissions for group
• permissions for other
• access checks made against process’s
effective ids
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Chmod command
• Symbolic access modes
• example: chmod +r file
• Octal access modes
octal
read
write
execute
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yes
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Directory permissions
• Same types and sets of permissions as for
files
– read: means process may a read a dir (i.e., list
files)
– write: process add/rm files in dir
– execute: process can “search”, access files, in
dir or subdir
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Common Utilities for Managing files and
directories
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pwd
cat, ed, vi, emacs…
ls
rm
mv
cp
ln
mkdir and rmdir
lp:
wc
print process current dir
create files
list contents of directory
remove file
rename file
copy a file
create a hard link to a file
create and remove dir
print a file
counts the words in a file
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Unix Processes
Definitions:
• program: collection of bytes and data
stored in a file
• image: computer execution environment
• process: execution of an image
• multi-tasking: many processes can execute
simultaneously in Unix.
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Unix Process Groups
• process id: unique id assigned to process
upon creation
• process group id: id of the group to which
the process belongs to
• foreground process group: is the process
group associated with a terminal at a time
• background process group: processes
created by you not in the foreground group
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Process Relationships
• A process spawns another process using the
fork(2) system call.
• The creating process is the parent process
• The newly created process is the child process.
• fork() returns 0 to the child process
• fork() returns the process_id of the child to the
parent process
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Process Relationship (continued)
• exec(2) :To run a new program, the child,
will issue the exec( ) system call and
overwrites itself with the code and initial
data of the new program, thus initiating the
execution of the new program
• wait(2): a parent can suspend its execution
until one or more child processes complete
via a wait(2) system call
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Process Relationships (continued)
• exit(2) :upon terminations, process can set
an exit status available to parent. Code used
– zero for success
– non-zero for failure
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Example: Program that creates a new process to copy files
main(argc,argv)
int(argcl
char *argv[];
{/* assumes 2 args, source and target files */
if ( fork() == 0) {
/* child process */
execl("cp"."cp",argv[1],argv[2],0);
}
/* parent process */
wait(int *) 0);
printf("copy done\n");
}
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Fork operation
After fork operation
shared text
parent process data
child process data
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After exec of prog2 in child
(prog2 is cp in example)
After exec "prog2" in child
parent process data
child process data
prog text data unchanged
prog2 text data
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Unix process genealogy
Process generation
Init process 1
forks init processes
init
execs
init
execs
Init
execs
getty
getty
execs
getty
login
execs
/bin/sh
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Process permissions
• real id and one of more real group id set at
login.
• effective uid and effective group id
determine process access to
read/write/search/execute files or dir.
• umask() file mode creation mask, used
when file or dir created by process
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Signals
• Signal: mesg a process can send to a process or
process group, if it has appropriate permissions.
• mesg number represented by a symbolic name
• for each signal, receiving process can:
– explicitly ignore signal
– specify action to be taken upron receipt (signal
handler)
– otherwise, default action takes place (usually
process is killed)
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Signals (continued)
Example:
• When a child exists, it send a SIGCHLD
signal to its parent.
• When the parent issues a wait, it tells the
system it wants to catch the SIGCHLD
signal
• When a parent does not issue a wait, it
ignores the SIGCHLD signal
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Inter-process Communication
Related Processes
• signals
• read/write regular files
• pipes: when a process B tries to read from a pipe
– returns data if process A has written to pipe
– returns with EOF, if no other process has pipe
open for writing
– suspends execution until process A writes data
to it
• child returns exit value to waiting parent process
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Interprocess Communication
Unrleated Processes
– FIFO (named Pipes)
– System V IPC
• msg queues
• semaphores
• shared memory
– sockets (client/server model)
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Process Environment includes:
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Process id and process group id
open files
current working directory
real and effective user and group ids
file creation mask (umask)
resource limits
signal action settings
set of named local variables
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File Descriptors
• each process associates a number or
handle, called file descriptor, (fd) with
each file it has opened.
• At login, three files associated with terminal
– standard input: fd 0, open for reading
– standard output: fd 1, open for writing
– standard error: fd 2, open for reading,writing
• process inherits parent’s file descriptors
unless specified (close-on-exec)
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Process Subsystem utilities
• ps
• kill
• wait
• nohup
• sleep
• nice
monitors status of processes
terminate a process (by pid)
parent process wait for one of its
children to terminate
makes a command immune to the
hangup and terminate signal
sleep in seconds
run processes at low priority
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Setuid and Setgid Mechanisms
• Mechanism pattented
• process effective uids are different from its
real uids when it executes a set-uid or setgid program.
• the process effective uid and gid become
that of the executable
• example: changing your passwd
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Security Problems
• Permissions on the executable program
• and directory in which it is contained must
be correct, otherwise easily replaced by
Trojan Horse.
• Some systems remove setuid and setgid bits
whenever files are modified as a security
precaution.
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Overview of the shell
• Command line interpreter and programming
language between operating system and
user
• user may select which shell to run:
– /bin/csh
– /bin/ksh
– other shells
Cshell
Korn shell
• shell scripts: files of UNIX and shell
commands executed from a UNIX shell
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Working with the shell
• Shell invoked automatically during a login
session or manually at the prompt by user
– 1. Reads a special startup file for initialization
– 2. Displays prompt and waits for user command
– 3. Executes user command and goes to step 2,
unless contrl D, then shell terminates
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Redirection of input/ouput
• Redirection of output: >, >>
– example:$ man ls > info.ls
• Redirection of input: <
– example: $ cat <input.data
• using filters: pipes
– example: $ cat file| wc -l;
/* counts the number of line in file */
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Shell Core Features
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Simple and complex commands
redirection of input/output
pipes
wildcards
command substitution
background processes
shell variables
here documents
built-in cmds
programming constructs
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Simple Commands supported
• simple command: sequence of non blanks
arguments separated by blanks or tabs.
• 1st argument (numbered zero) usually specifies the
name of the command to be executed.
• Any remaining arguments (with a few exceptions,
see meta-characters)
– Are passed as arguments to that command.
– Arguments may be filenames, pathnames, directories or
special options
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Complex commands
• Multiple commands
• Command groupings
• Conditional command execution
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File name expansion
• Wildcards
* matches any string of characters
? matches any single character
[list] matches any character in list
[lower-upper] matches any character in range
lower-upper inclusive
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Shell Scripts
• A shell script is a regular text file that contains
shell or UNIX commands
• Before running it , it must have execute
permissions ( see chmod +x filename)
• Very useful for automating repetitive task and
administrative tools and for storing commands for
later execution
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Shell Scripts (continued)
• When a script is run , kernel determines which
shell it is written for by examining the first line of
the script
– If 1st line is just #, then it is interpreted by a C
shell
– If 1st line is of the form #!pathname, then the
executable
– Pathname is used to interpret the script
– If neither rule 1 nor rule 2 applies, the script is
interpreted by a Bourne shell.
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Here Documents
• Shell provides alternative ways of supplying
standard input to commands
• Shell allows in-line input redirection using <<
called here documents
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command [arg(s)] << arbitrary-delimiter
command input
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arbitrary-delimiter
• arbitrary-delimiter should be a string
that does not appear in text
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Shell Variables
• Shell has several mechanisms for creating
variables. A variable is a name
• Representing a string value
– Shell variables can save time and reduce typing errors,
variables
• Allow you to store and manipulate information
• two types: local and environmental
– local are set by the user of by the shell itself
– Positional parameters variables are normally set only on
a command line
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Environmental Variables
NAME
$HOME
directory
$PATH
$MAIL
$USER
$SHELL
$TERM
MEANING
absolute pathname of your home
a list of directories to search for
absolute pathname to mailbox
your user id
absolute pathname of login shell
type of your terminal
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Positional parameters
• when a shell procedure is invoked, the shell
implicitly creates positional parameters. The name
for a positional parameter is a number.
• Positional parameters are used mainly in scripts.
– $0 is the argument in position zero on the command
line
– $1 is the first argument
– $1.. $9 $n refers to the nth argument on the command
line if applicable
– $# the number of positional parameters, not counting 0
– $* the list of all arguments
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QUOTING
• Quoting restores the literal meaning to
characters that are processed specially by the
shell. The literal quotes are not passed on to
the command
• Single quotes ( ' ) inhibit wildcard
replacement, variable substitution, and
command substitution
• Double quotes ( " ) inhibit wildcard
replacement only
• When quotes are nested, only the outer
quotes have any effect
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BUILT-IN commands
• commands that are internal to the shell
• Faster to execute and more efficient than
other commands
– Shell does not have to fork to execute the
command
– Trade-off: redirection of input/output not
allowed for most of these
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Built-in commands (continued)
• built-in commands common to the 3 shells:
echo
cd
wait
exit
exec
shift
umask
eval
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End of Lecture
• Questions?
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Subshells
• When a parent shell forks a child to execute a
command, the new child shell is sometimes called
a subshell. This happens when:
– a group command is executed ( $(cmd1; cmd2;
cmd3) )
– a shell script is executed ( $myscript )
– a background job is executed ( cmd1&)
• A shell inherits the parent's environment but not
the parent's local variables.
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