Lecture 2
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Log into Linux
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Questions?
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Homeworks 1 & 2 posted
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Homework 1 due Tuesday next week
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Homework 2 due Thursday next week
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CS 375 UNIX System Programming - Lecture 2
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Outline
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Files and directories
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UNIX philosophy
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Essential commands
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grep
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find
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File Information
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Use “ls -l” to display complete file info:
$ ls ­l
drwxrwxr­x 11 hwang faculty 4096 Jul 25 10:43 qt
­rw­r­­r­­ 1 hwang faculty 6455 Aug 19 14:53 readme.txt
lrwxrwxrwx 1 hwang faculty 23 Aug 8 12:59 rt ­> a.txt
prw­rw­r­­ 1 hwang faculty 0 Aug 27 15:18 mypipe
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Shown: file type, permissions, # of hard links,
owner, group, size, last modification time, and
name.
File type is indicated by first character
­ Regular file
p Named pipe
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d Directory l Soft link c Char device b Block device CS 375 UNIX System Programming - Lecture 2
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Soft (Symbolic) LInks
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Use “ln -s” to create soft or symbolic links. A
soft link is a pointer to a “real name” for a file.
These are similar to Windows shortcuts but are
supported by both the CLI and the GUI.
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$ ln ­s target linkname
The source file may be on another filesystem.
You can create soft links to directories.
Deleting the link has no effect on the file.
Deleting the file can leave a soft link to nothing. I.e.
a "dangling reference".
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Hard Links
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Use ln to create hard links. Creating a hard link
creates another “real name” for a file.
$ ln target newname ●
Data is not deleted until all hard links are deleted.
Names are aliases, so changes are reflected.
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Data must be on the same filesystem as the link.
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System call to delete a file is actually “unlink”.
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Only the administrator can create hard links to
directories (to prevent filesystem corruption due to
directory “loops” by unwitting users) .
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File Links
name1
DATA
name2
File with two hard links
name1
DATA
name2
File with one hard link and one soft link
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Special File Names
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A filename whose first character is a period will
not show up in normal directory listing.
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Use 'ls -a' to list hidden files
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This has NOTHING to do with security.
Every directory automatically includes . and ..
which are links to itself and its parent directory.
$ cp /etc/passwd .
$ cd ../../../../etc
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Device Files
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Device files provide access to hardware. There
are two types: character and block
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They reside in the /dev directory.
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They can be created with mknod by the administrator.
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Permissions determine access to corresponding
hardware.
Character devices provide sequential access (serial
ports) while block devices are used for random access
(drives)
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Device Files
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See the makedev man page and the
devices.txt file in the kernel source archive for
a list of devices. See also man pages for hd
(hex dump), od (octal dump), random, etc.
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A Few Standard Device Files
/dev/hda
/dev/hdb
/dev/hdc
/dev/hdd
/dev/cdrom
/dev/sda
/dev/sr0
/dev/st0
/dev/hda1
/dev/hda2
/dev/hda5
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Master IDE drive on first controller
Slave IDE drive on first controller
Master IDE drive on second controller
(often is CDROM/DVD drive)
Slave IDE drive on second controller (often was used for ZIP drive)
Link to CDROM device
First SCSI drive
CDROM device
Tape drive
First primary partition on hda drive
(C: drive in Windows ­ partitioning
sda is similar)
Second primary partition on sda drive
First logical partition
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A Few Standard Device Files
/dev/tty
/dev/tty1
/dev/pts/1
/dev/fd/0
/dev/stdin
/dev/ttyS0
/dev/lp0
/dev/audio
/dev/null
/dev/zero
/dev/random
/dev/mem
/dev/psaux
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Current terminal device
First console device
First pseudo­terminal (xterm) device
File descriptor 0 or standard input
(1 is stdout, 2 is stderr)
Link for /dev/fd/0 (also /dev/stdout,
/dev/stderr)
First serial port (COM1)
First parallel port
Sound card i/o
Null device (bit bucket)
Zero device (all zeroes)
Random number generator
Physical memory
PS/2 mouse port
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Cool Device Tricks
$ dd if=/dev/cdrom of=cdrom.iso # Create ISO image of CD
$ cat /dev/audio > sound.au
# Record from microphone
$ cat sound.au > /dev/audio
# Play sound
$ program 2> /dev/null
# Redirect program error
# messages to bit bucket
$ program /dev/stdin # Pass stdin as filename argument
$ od ­A n ­N 8 ­t u2 < /dev/random # Generate 4 16­bit unsigned random #s
You can create, format, and mount disk images!
$ dd if=/dev/zero of=f.img bs=1k count=1440
# Create 1.44MB zero file
$ mkfs ­t msdos f.img
# Create a file system
$ mkdir a
# Create a mount point
$ mount ­o loop f.img a
# Mount the file system
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Cool Device Tricks
// File: random.cpp
// Print 10 random 32­bit integers
#include <fstream>
#include <iostream>
using namespace std;
int main()
{
int randnum;
ifstream randstr("/dev/random");
for(int count=0; count<10; count++) {
randstr.read((char *)&randnum,sizeof(randnum));
cout << count << ": " << randnum << endl;
}
randstr.close();
return 0;
}
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Filesystems
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Disk devices (e.g. /dev/sda1) contain raw disk
data that is not normally accessed directly by
the user.
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A filesystem contains inode lists and data blocks.
An inode structure (on disk) contains all file info except the
file name (owner, group, perms, times, link count, and
indexes to the data blocks that make up the file).
A directory block has only filename and inode # pairs.
Use “ls -i” to display inode numbers.
Use mkfs (as administrator) to create a filesystem (format):
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$ mkfs ­t ext3 /dev/sdc1
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Filesystems
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All files in a UNIX systems are arranged in a
tree rooted at /.
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No C:, D:, etc drives as in Windows.
Easily add disk space and allow files to maintain the
same position in directory tree.
mount is used to attach a filesystem to the tree
$ mount ­t ext3 /dev/sdc1 /home
$ mount /dev/fd0 /media/floppy
$ mount /dev/hdc /media/cdrom
$ mount
# Display mounted filesystems
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Filesystems
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List of automatically mounted devices is in
/etc/fstab
Use df (diskfree) to display the amount of
freespace on each filesystem.
umount (not unmount) detaches a filesystem
Many systems have an automounter program
running to automount removable media (USB
sticks, CD-ROMs, etc.)
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Standard Directory Tree
/
/bin
/lib
/sbin
/etc
/home
/root
/dev
/var
/tmp
/usr/bin
/usr/lib
/usr/local
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root directory (don't confuse with /root)
essential utilities
essential libraries
essential admin tools
configuration files
contains user HOME directories
root user HOME directory
device directory
system files that change (log, spool files)
for temp file usage (avail to all users)
user applications
user application libraries
contains applications added by local admin
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File Permissions
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Regular file permissions are specified for user
(owner), group and other (anyone else).
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Permissions are r (read), w (write) and x (execute)
Write permission allows you to alter a file's contents. It
does not give you permission to delete the file.
Permissions also apply to named pipes and devices.
Here are some examples:
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­rw­r­­r­­ User can read and write, group other just read
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­rwxr­xr­x All can read and execute, user can write
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Directory Permissions
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Permissions apply to user, group and other.
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Read implies permission to get a directory listing.
Write indicates permission to create and delete files in the
directory. (Write permission on the file is not required!)
Execute means permission to access files in the directory.
This makes more sense if you think of a directory as a file
that contains filenames. (Actually, that's what it is.)
Here are some examples:
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drwxr­xr­x User has full access, others can use ls, cp
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drwxrwx­­­ User and group have full access, others none
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Changing Permissions
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Use the chmod command
You must be the owner (or root) to change
permissions.
Symbolic mode examples
$ chmod u=rw,og=r readme.txt
$ chmod uog+rx datadir
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Numeric mode (r=4, w=2, x=1)
$ chmod 644 readme.txt
$ chmod 755 datadir
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Additional Permission Info
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Special permissions: setuid, setgid, sticky. See
'man chmod' and 'info “File Permissions”'
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­rwsr­xr­x setuid (run as owner), chmod u+s
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­rwxrwsr­x setgid (run as group), chmod g+s
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drwxrwxrwt sticky (only allow owner to delete), o+t
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Look at perms on /usr/bin/chsh (setuid) and /tmp (sticky)
Linux also supports ACLs (access control lists)
for fine-grained control, i.e. to give individual
users special access to directories and files.
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File Ownership
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Use chown to change file ownership
$ chown newusername readme.txt
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You must be administrator
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Newusername becomes the owner of files
Use chgrp to change file group
$ chgrp newgroupname readme.txt
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You must belong to both old group and new group
Can use chown to change both at once
$ chown newusername.newgroupname readme.txt
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File Timestamps
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There are three times associated with a file: last
access, modification, and change
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Modification time is when file data was altered.
Change time is when owner, group, permission, or
hard link count was altered.
“ls -l” shows modification time by default.
Use “ls -l --time=atime” or “ls -l --time=ctime” to see
others
You can update access and modification times
using touch.
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UNIX Philosophy
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Here are a few notes on UNIX programming
philosophy:
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Write small programs that do one thing well.
Read data from standard input (scanf, cin) and write
data to standard output (printf, cout). Such
programs are known as filters. Very complex
applications can be created combining filters using
pipes.
Move core routines into well-documented libraries
so that they can be reused.
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Essential Commands
COMMAND
DESCRIPTION
ls
Displays directory listing
cd directory
Change the current (working) directory
cd
Return to HOME directory
pwd Display the current directory
mkdir dirname Create a directory
file filename Display file type
cat textfile Display file contents
less textfile Page file contents to display (also more)
exit or ^D
Leave this session
passwd
Change your password
man command Display man pages on command
info command Display info pages on command
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Useful Commands
COMMAND
DESCRIPTION
apropos string Search the whatis (manual page) database
for string (same as “man ­k string”)
whatis command Display brief description from whatis
database (same as “man ­f command”)
command ––help Display brief help on command
ps aux
Display all processes
kill
Kill a process (requires ownership)
du foo
Display disk usage by file or dir foo
df
Display free disk space
grep
Search for character strings
find
Find files meeting criteria
tar
Standard UNIX archive tool
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grep
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general regular expression parser
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Used to find strings inside files; prints file name and
line by default
Syntax:
grep [options] pattern [files]
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Options change the way command behaves
-c
-i
-l
-v
-E
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count lines that match
ignore case
list names of files w/o line
invert matching, selects non-matching lines
extended expressions (slower)
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Regular Expressions
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Patterns use regular expressions
^
$
.
[]
?
*
+
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anchor to beginning of line
anchor to end of line
match any single character
match one of a set of chars, can be a range, prefix with ^
for inverted match
optional - match 0 or 1 time
match 0 or more times
match 1 or more times
Special match patterns inside [ ]
[:alnum:]
[:alpha:]
[:digit:]
[:space:]
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alphanumeric chars
letters
digits
whitespace
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grep
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Examples:
$
$
$
$
$
cd /usr/share/dict/
grep in words
grep ^in words
grep in$ words
grep ^in[[:alpha:]]in$ words
$ grep ^[cd] words
$ grep ^[^a]*$ words
$ grep ^[^ae]*$ words
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#
#
#
#
contains "in"
begin with "in"
end with "in"
begin and end with "in"
# begin with c or d
# no a's
# no a's or e's
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find
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Syntax:
find [path] [options] [tests] [actions]
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Search for files starting at path. Often / or .
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Main options; always return true
-follow
-maxdepths N
-mount
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Follow symbolic links
Search at most N levels
Don't search other file systems
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find
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Tests return true or false. If false, stops. If true,
does next test/action
-atime N
last accessed N days ago;
+N for greater than;
-N for less than
-mtime N
last modified N days ago
-name pattern file name matches pattern;
must be in quotes
-newer otherfile file is newer than otherfile
-type C
file is type C – e.g. f for regular file
-user username file is owned by username
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find
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Combine tests with logical connectives
!, -not
-a, -and
-o, -or
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invert test
both test must be true
one test must true
Use escaped parentheses to group
Actions are at the end and say what to do with
each file that matches
-print
-ls
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print the name of matching file
use ls -dils on matching file
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find
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Any command can be executed
-exec command
execute command on match
-ok command
ask user confirmation before
executing command on match
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These actions must end with \; sequence in
order for find to know where the embedded
command ends
Use { } to represent the matching file
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find
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Examples:
$ find / -name test -print
# find all files named "test"
$ find . -mtime 2 -print
# modified exactly 2 days ago
$ find . -mtime -2 -type f -print
# regular files modified in last 2 days
$ find . \( -mtime -2 -o -atime -2 \) -print
# modified or accessed in last 2 days
$ find . \( -mtime -7 -a -name "*.cpp" \) -print
# C++ source files modified in last week
$ find . \( -mtime -7 -a -name "*.cpp" \) -exec ls -l {} \;
# execute ls -l on them
$ find . \( -mtime -7 -a -name "*.cpp" \) \
-exec grep ^int {} \;
# print out lines starting with "int"
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