Basic Unix tools:
1.
Explain the difference between these two commands. This question is very
important.
If you don't know the answer, then look back at the shell chapter.
find
/data -name "*.txt"
find
/data -name *.txt
When
*.txt is quoted then the shell will not touch it. The find tool will look in
the
/data for
all files ending in .txt.
When *.txt
is not quoted then the shell might expand this (when one or more files
that ends
in .txt exist in the current directory). The find might show a different
result,
or can
result in a syntax error.
2.
Explain the difference between these two statements. Will they both work when
there are
200 .odf files in /data ? How about when there are 2 million .odf files ?
find
/data -name "*.odf" > data_odf.txt
find
/data/*.odf > data_odf.txt
The first
find will output all .odf filenames in /data and all subdirectories. The shell
will
redirect this to a file.
The
second find will output all files named .odf in /data and will also output all
files
that
exist in directories named *.odf (in /data).
With two
million files the command line would be expanded beyond the maximum
that the
shell can accept. The last part of the command line would be lost.
3. Write
a find command that finds all files created after January 30th 2010.
touch -t
201001302359 marker_date
find .
-type f -newer marker_date
There is
another solution :
find .
-type f -newerat "20100130 23:59:59"
4. Write
a find command that finds all *.odf files created in September 2009.
touch -t
200908312359 marker_start
touch -t
200910010000 marker_end
find .
-type f -name "*.odf" -newer marker_start ! -newer marker_end
The
exclamation mark ! -newer can be read as not newer.
5. Count
the number of *.conf files in /etc and all its subdirs.
find /etc
-type f -name '*.conf' | wc –l
6. Two
commands that do the same thing: copy *.odf files to /backup/ . What would
be a
reason to replace the first command with the second ? Again, this is an
important
question.
cp -r
/data/*.odf /backup/basic Unix tools
144
find
/data -name "*.odf" -exec cp {} /backup/ \;
The first
might fail when there are too many files to fit on one command line.
7. Create
a file called loctest.txt. Can you find this file with locate ? Why not ? How
do you
make locate find this file ?
You
cannot locate this with locate because it is not yet in the index.
Updated
8. Use
find and -exec to rename all .htm files to .html.
paul@rhel55
~$ find . -name '*.htm'
./one.htm
./two.htm
paul@rhel55
~$ find . -name '*.htm' -exec mv {} {}l \;
paul@rhel55
~$ find . -name '*.htm*'
./one.html
./two.html
9. Issue
the date command. Now display the date in YYYY/MM/DD format.
date
+%Y/%m/%d
10. Issue
the cal command. Display a calendar of 1582 and 1752. Notice anything
special ?
cal 1582
files/studentfiles/dates.txt
Working with directories:
1.
Display your current directory.
pwd
2. Change
to the /etc directory.
cd /etc
3. Now
change to your home directory using only three key presses.
cd (and
the enter key)
4. Change
to the /boot/grub directory using only eleven key presses.
cd
/boot/grub (use the tab key)
5. Go to
the parent directory of the current directory.
cd ..
(with space between cd and ..)
6. Go to
the root directory.
cd /
7. List
the contents of the root directory.
ls
8. List a
long listing of the root directory.
ls -l
9. Stay
where you are, and list the contents of /etc.
ls /etc
10. Stay
where you are, and list the contents of /bin and /sbin.
ls /bin
/sbin
11. Stay
where you are, and list the contents of ~.
ls ~
12. List
all the files (including hidden files) in your home directory.
ls -al ~
13. List
the files in /boot in a human readable format.
ls -lh
/boot
14.
Create a directory testdir in your home directory.
mkdir
~/testdir
15.
Change to the /etc directory, stay here and create a directory newdir in your
home
directory.working
with directories
34
cd /etc ;
mkdir ~/newdir
16.
Create in one command the directories ~/dir1/dir2/dir3 (dir3 is a subdirectory
from
dir2, and dir2 is a subdirectory from dir1 ).
mkdir -p
~/dir1/dir2/dir3
17.
Remove the directory testdir.
rmdir
testdir
18. If
time permits (or if you are waiting for other students to finish this
practice),
use and
understand pushd and popd. Use the man page of bash to find information
about
these commands.
man bash
paul@laika:/etc$
cd /bin
paul@laika:/bin$ pushd /lib
/lib /bin
paul@laika:/lib$ pushd /proc
/proc /lib /bin
paul@laika:/proc$
paul@laika:/proc$ popd
/lib /bin
paul@laika:/lib$
paul@laika:/lib$
paul@laika:/lib$ popd
/bin
paul@laika:/bin$
Working with files:
1. List
the files in the /bin directory
ls /bin
2.
Display the type of file of /bin/cat, /etc/passwd and /usr/bin/passwd.
file
/bin/cat /etc/passwd /usr/bin/passwd
books/LinuxFun.pdf)
3b.
Display the type of file of wolf.jpg and LinuxFun.pdf
file
wolf.jpg LinuxFun.pdf
3c.
Rename wolf.jpg to wolf.pdf (use mv).
mv
wolf.jpg wolf.pdf
3d.
Display the type of file of wolf.pdf and LinuxFun.pdf.
file
wolf.pdf LinuxFun.pdf
4. Create
a directory ~/touched and enter it.
mkdir
~/touched ; cd ~/touched
5. Create
the files today.txt and yesterday.txt in touched.
touch
today.txt yesterday.txt
6. Change
the date on yesterday.txt to match yesterday's date.
touch -t
200810251405 yesterday.txt (substitute 20081025 with yesterday)
7. Copy
yesterday.txt to copy.yesterday.txt
cp
yesterday.txt copy.yesterday.txt
8. Rename
copy.yesterday.txt to kim
mv
copy.yesterday.txt kim
9. Create
a directory called ~/testbackup and copy all files from ~/touched into it.
mkdir
~/testbackup ; cp -r ~/touched ~/testbackup/
10. Use
one command to remove the directory ~/testbackup and all files into it.
rm -rf
~/testbackup
11.
Create a directory ~/etcbackup and copy all *.conf files from /etc into it. Did
you
include
all subdirectories of /etc ?
File contents:
1.
Display the first 12 lines of /etc/services.
head -12
/etc/services
2.
Display the last line of /etc/passwd.
tail -1
/etc/passwd
3. Use
cat to create a file named count.txt that looks like this:
cat >
count.txt
One
Two
Three
Four
Five
(followed by Ctrl-d)
4. Use cp
to make a backup of this file to cnt.txt.
cp
count.txt cnt.txt
5. Use
cat to make a backup of this file to catcnt.txt.
cat
count.txt > catcnt.txt
6.
Display catcnt.txt, but with all lines in reverse order (the last line first).
tac
catcnt.txt
7. Use
more to display /var/log/messages.
more
/var/log/messages
8.
Display the readable character strings from the /usr/bin/passwd command.
strings
/usr/bin/passwd
9. Use ls
to find the biggest file in /etc.
ls -lrS
/etc
10. Open
two terminal windows (or tabs) and make sure you are in the same directory
in both.
Type echo this is the first line > tailing.txt in the first terminal, then
issue
tail -f
tailing.txt in the second terminal. Now go back to the first terminal and type
echo This
is another line >> tailing.txt (note the double >>), verify that
the tail -f
in the
second terminal shows both lines. Stop the tail -f with Ctrl-C.
11. Use
cat to create a file named tailing.txt that contains the contents of
tailing.txt
followed
by the contents of /etc/passwd.
cat
/etc/passwd >> tailing.txt
12. Use
cat to create a file named tailing.txt that contains the contents of
tailing.txt
preceded
by the contents of /etc/passwd.
mv
tailing.txt tmp.txt ; cat /etc/passwd tmp.txt > tailing.txt
File system tree:
1. Does
the file /bin/cat exist ? What about /bin/dd and /bin/echo. What is the type
of these
files ?
ls
/bin/cat ; file /bin/cat
ls /bin/dd ; file /bin/dd
ls /bin/echo ; file /bin/echo
2. What
is the size of the Linux kernel file(s) (vmlinu*) in /boot ?
ls -lh
/boot/vm*
3. Create
a directory ~/test. Then issue the following commands:
cd ~/test
dd
if=/dev/zero of=zeroes.txt count=1 bs=100
od
zeroes.txt
dd will
copy one times (count=1) a block of size 100 bytes (bs=100) from the file /
dev/zero
to ~/test/zeroes.txt. Can you describe the functionality of /dev/zero ?
/dev/zero is a Linux special device. It can be considered a source
of zeroes. You
cannot
send something to /dev/zero, but you can read zeroes from it.
4. Now
issue the following command:
dd
if=/dev/random of=random.txt count=1 bs=100 ; od random.txt
dd will
copy one times (count=1) a block of size 100 bytes (bs=100) from the file /
dev/random
to ~/test/random.txt. Can you describe the functionality of /dev/random
?
/dev/random
acts as a random number generator on your Linux machine.
5. Issue
the following two commands, and look at the first character of each output
line.
ls -l
/dev/sd* /dev/hd*
ls -l
/dev/tty* /dev/input/mou*
The first
ls will show block(b) devices, the second ls shows character(c) devices. Can
you tell
the difference between block and character devices ?
Block
devices are always written to (or read from) in blocks. For hard disks, blocks
of 512
bytes are common. Character devices act as a stream of characters (or bytes).
Mouse and
keyboard are typical character devices.
6. Use
cat to display /etc/hosts and /etc/resolv.conf. What is your idea about the
purpose
of these files ?the Linux file tree
71
/etc/hosts
contains hostnames with their ip address
/etc/resolv.conf
should contain the ip address of a DNS name server.
7. Are
there any files in /etc/skel/ ? Check also for hidden files.
Issue
"ls -al /etc/skel/". Yes, there should be hidden files there.
8.
Display /proc/cpuinfo. On what architecture is your Linux running ?
The file
should contain at least one line with Intel or other cpu.
9.
Display /proc/interrupts. What is the size of this file ? Where is this file
stored ?
The size
is zero, yet the file contains data. It is not stored anywhere because /proc is
a virtual
file system that allows you to talk with the kernel. (If you answered
"stored
in
RAM-memory, that is also correct...).
10. Can
you enter the /root directory ? Are there (hidden) files ?
Try
"cd /root". Yes there are (hidden) files there.
11. Are
ifconfig, fdisk, parted, shutdown and grub-install present in /sbin ? Why are
these
binaries in /sbin and not in /bin ?
Because
those files are only meant for system administrators.
12. Is
/var/log a file or a directory ? What about /var/spool ?
Both are
directories.
13. Open
two command prompts (Ctrl-Shift-T in gnome-terminal) or terminals (CtrlAlt-F1,
Ctrl-Alt-F2, ...) and issue the who am i in both. Then try to echo a word from
one
terminal to the other.
tty-terminal: echo Hello > /dev/tty1
pts-terminal: echo Hello > /dev/pts/1
14. Read
the man page of random and explain the difference between /dev/random
and
/dev/urandom.
man 4
random
Redirection and pipes:
1. Use ls
to output the contents of the /etc/ directory to a file called etc.txt.
ls /etc
> etc.txt
2.
Activate the noclobber shell option.
set -o
noclobber
3. Verify
that nocclobber is active by repeating your ls on /etc/.
ls /etc
> etc.txt (should not work)
4. When
listing all shell options, which character represents the noclobber option ?
echo $-
(noclobber is visible as C)
5.
Deactivate the noclobber option.
set +o
noclobber
6. Make
sure you have two shells open on the same computer. Create an empty
tailing.txt
file. Then type tail -f tailing.txt. Use the second shell to append a line of
text to
that file. Verify that the first shell displays this line.
paul@deb503:~$ > tailing.txt
paul@deb503:~$ tail -f tailing.txt
hello
world
in the
other shell:
paul@deb503:~$ echo hello >> tailing.txt
paul@deb503:~$
echo world >> tailing.txt
7. Create
a file that contains the names of five people. Use cat and output redirection
to create
the file and use a here document to end the input.
paul@deb503:~$
cat > tennis.txt << ace
>
Justine Henin
>
Venus Williams
>
Serena Williams
>
Martina Hingis
> Kim
Clijsters
> ace
paul@deb503:~$
cat tennis.txt
Justine
Henin
Venus
Williams
Serena
Williams
Martina
Hingis
Kim
Clijsters
paul@deb503:~$
***Linux security concepts(Groups & users):***
Groups:
1. Create
the groups tennis, football and sports.
groupadd
tennis ; groupadd football ; groupadd sports
2. In one
command, make venus a member of tennis and sports.
usermod
-a -G tennis,sports venus
3. Rename
the football group to foot.
groupmod
-n foot football
4. Use vi
to add serena to the tennis group.
vi
/etc/group
5. Use
the id command to verify that serena is a member of tennis.
id (and
after logoff logon serena should be member)
6. Make
someone responsible for managing group membership of foot and sports.
Test that
it works.
gpasswd
-A (to make manager)
gpasswd
-a (to add member)
Users:
1. Create
the users Serena Williams, Venus Williams and Justine Henin, all of them
with
password set to stargate, with username (lower case) as their first name, and
their
full name in the comment. Verify that the users and their home directory are
properly
created.
useradd
-m -c "Serena Williams" serena ; passwd serena
useradd -m -c "Venus Williams" venus ; passwd
venus
useradd -m -c "Justine Henin" justine ; passwd
justine
tail /etc/passwd ; tail /etc/shadow ; ls /home
Keep user
logon names in lowercase!
2. Create
a user called kornuser, give him the Korn shell (/bin/ksh) as his default
shell.
Log on with this user (on a command line or in a tty).
useradd
-s /bin/ksh kornuser ; passwd kornuser
3. Create
a user named einstime without home directory, give him /bin/date as his
default
logon shell. What happens when you log on with this user ? Can you think of
a useful
real world example for changing a user's login shell to an application ?
useradd -s /bin/date einstime ; passwd einstime
It can be
useful when users need to access only one application on the server. Just
logging
on opens the application for them, and closing the application automatically
logs them
off.
4. Try
the commands who, whoami, who am i, w, id, echo $USER $UID .
who ;
whoami ; who am i ; w ; id ; echo $USER $UID
5a. Lock
the venus user account with usermod.
usermod
-L venus
5b. Use
passwd -d to disable the serena password. Verify the serena line in /etc/
shadow
before and after disabling.
grep
serena /etc/shadow; passwd -d serena ; grep serena /etc/shadow
5c. What
is the difference between locking a user account and disabling a user
account's
password ?
Locking
will prevent the user from logging on to the system with his password (by
putting a
! in front of the password in /etc/shadow). Disabling with passwd will erase
the
password from /etc/shadow.
6. As
root change the password of einstime to stargate.
Log on as
root and type: passwd einstime
7. Now
try changing the password of serena to serena as serena.
log on as
serena, then execute: passwd serena... it should fail!
file permissions:
1. As
normal user, create a directory ~/permissions. Create a file owned by yourself
in there.
mkdir
~/permissions ; touch ~/permissions/myfile.txt
2. Copy a
file owned by root from /etc/ to your permissions dir, who owns this file
now ?
cp
/etc/hosts ~/permissions/
The copy
is owned by you.
3. As
root, create a file in the users ~/permissions directory.
(become
root)# touch /home/username/permissions/rootfile
4. As
normal user, look at who owns this file created by root.
ls -l
~/permissions
The file
created by root is owned by root.
5. Change
the ownership of all files in ~/permissions to yourself.
chown
user ~/permissions/*
You
cannot become owner of the file that belongs to root.
6. Make
sure you have all rights to these files, and others can only read.
chmod 644
(on files)
chmod 755
(on directories)
7. With
chmod, is 770 the same as rwxrwx--- ?
yes
8. With
chmod, is 664 the same as r-xr-xr-- ?
No
9. With
chmod, is 400 the same as r-------- ?
yes
10. With
chmod, is 734 the same as rwxr-xr-- ?
no
11a.
Display the umask in octal and in symbolic form.
umask ;
umask –Ss
11b. Set
the umask to 077, but use the symbolic format to set it. Verify that this
works.
umask -S
u=rwx,go=
12.
Create a file as root, give only read to others. Can a normal user read this
file ?
Test
writing to this file with vi.
(become
root)
# echo
hello > /home/username/root.txt
# chmod
744 /home/username/root.txt
(become
user)
vi
~/root.txt
13a.
Create a file as normal user, give only read to others. Can another normal user
read this
file ? Test writing to this file with vi.
echo
hello > file ; chmod 744 file
Yes,
others can read this file
13b. Can
root read this file ? Can root write to this file with vi ?
Yes, root
can read and write to this file. Permissions do not apply to root.
14.
Create a directory that belongs to a group, where every member of that group
can read
and write to files, and create files. Make sure that people can only delete
their own
files.
mkdir
/home/project42 ; groupadd project42
chgrp
project42 /home/project42 ; chmod 775 /home/project42
You can
not yet do the last part of this exercise...
