Unix/Linux basics
0011
Operating systems lab
Gergely Windisch
windisch.gergely@nik.bmf.hu
room 4.12
http://nik.bmf.hu/gwindisch/os_2010
Permissions (quiz)
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touch file1
chmod 354 file1
chmod a-X file1
chmod ug+r file1
chmod o-w file1
chmod g-w file1
What is the result?
Octal number?
Textual representation?
inode (quiz)
• Let's assume that we have two disks. Disk1 and Disk2. Disk1
holds /, Disk2 holds /home.
• Assume that the inode numbers increase by one each and
every time a new inode is created. The next free inode number
is 345 on Disk1 and 763 on Disk2.
• We run the following commands:
cd ~
echo "inodes rule" > ~/truth
cp truth t2
cp truth t3
mv t2 t4
ln t5 t3
ln t4 /etc/t6
mv /etc/t6 ~/t7
ln /etc/t7 t8
What are the inode numbers for truth, t2, t3, t4, t5, t6, t7 t8?
Control structures
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if [ logical_expression ]
then
...commands...
elif [ logical_expression_2 ]
then
...commands...
else
...commands...
fi
Logical expressions
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if [ -r filename ] : switches to analize files
-r : see if file exists and readable
-w: file exists and writeable
... man test
Comparing numbers: -eq instead of ==
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$n1 -eq $n2
$n1 -ne $n2 , gt, ge, lt, le
Logical expressions 2
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Comparing strings
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if [ ”string1” == ”string2” ]
then
echo...
fi
string1 and 2 could be $variable1 and
$variable2
white spacing matters!!! spaces must be
places around [, == and ].
Case
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case ”variable” in
”string1”) commands;;
”string2”) commands;;
*) commands;;
esac
apple=1
case ”$apple” in
”1”) ... ;;
”2”) ... ;;
esac
for
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for iteration_variable in list
do
commands
done
cycle steps over the elements of lists
list: "one two three four five six seven"
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a list of words divided by SPACE
can be written as a string or generated by a
command
for (continued)
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In BASH scripts we usually use for to iterate
over lists of files
use seq to have a "convetional" for loop
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for i in `seq 30`
do
commands
done
seq 10 returns: 1 2 3 4 5 6 7 8 9 10
Until, while
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until [ ”$K” -eq ”3” ]; do
commands
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done
while [ ”$K” -ne ”3” ]; do
commands
done
Input from user
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read
read K
read -n1 K : takes only one character (without
return)
Exercise 1

Create a shell script which takes a number as
an input parameter and then writes back in
English the „name” of that number. If the
given number is not between 0 and 9, it
should say: out of bounds (or any other error
message you find amusing).
Solution
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#!/bin/bash
echo ”You have given me: ”
case ”$1” in
”1”) echo ”one”;;
”2”) echo ”two”;;
...
*) echo ”too large”
esac
exit 0
Exercise 2
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Create a shell script that shows a list of
options to the user. If the user presses 1, the
program should list contents of the current
working directory. Pressing two tells the user
the name of the current directory, and if the
user presses 3, the program should quit. There
should be an error message for any other
keys.
Solution
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#!/bin/bash
K=0
echo '**********************'
echo '* Menu
*'
echo '* 1 - Pritnt name
*'
echo '* 2 - Contents *'
echo '* 3 - Quit
*'
echo '**********************'
echo Please choose one:
read K
case "$K" in
"1") ls -l
uptime;;
"2") pwd;;
"3") echo Bye!;;
*) echo "Choose one between 1 and 3";;
esac
Exercise 3

Create a shell script that takes the name of a file as
an input parameter and a string that should be
inserted in that file. The string can be omitted, in
that case the program should ask for it from the
user. Once it has the name of the file and the
string, it should append the string to the end of
the file, but only if the said file exists, is a text file
and is writeable. If not, give a customized error
message. If the program is started without
parameters, it should print usage information.
Hints for Exercise 3
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first input parameter: $1
second input parameter: $2
number of input parameters: $#
asking the user something: read varname
man test to find out how to check for
writeable files
appending example: echo "$something" >> file
Exercise
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Create a program which prints the content of
all the files in the current directory. Use for.
Solution
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for i in `ls`
do
cat $i
done
Exercise
Create a shell script that prints out all the
input parameters the user has entered in the
form of:
parameter 1 is: one
parameter 2 is: two etc.
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Exercise
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Create a user friendly shell script that requires
an input string which should be appended to a
file. The file name and the string should come
as parameters. If -v is an input parameter, it
should do it's job verbosely (tell the user what
it is doing at any given point). If -h is given,
then don't do anything but return some
helpful information (the same effect should
come when there are fewer than two
parameters).
Exercise
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Write a shell script that prints the content of
the current directory in the form of:
”File: name_of_file”
(bonus points: try to separate directories from
files)
Solution
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for i in `ls`
do
echo ”File: $i”
done
Exercise
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Create a shell script that copies the contents
of all the ASCII text files into one big file.
Hint
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use the file command to get the type of the
file
Solution
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#!/bin/bash
allthetext="newfile"
echo `date` > $allthetext
for i in `ls`
do
something=`file $i`
something2="$i: ASCII text"
if [ "$something" == "$something2" ]
then
if [ "$allthetext" != "$i" ]
then
# echo "$i is a text file"
cat $i >> $allthetext
fi
fi
done
Exercise
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Add commands to the program with the
menu, so that it really wouldn't quit unless the
user presses 3
Solution
#!/bin/bash
K=0
until [ "$K" -eq "3" ]; do
echo '**********************'
echo '* Menu
echo '* 1 - List
*'
*'
echo '* 2 - Current dir *'
echo '* 3 - Quit
*'
echo '**********************'
echo Please choose one:
read K
case "$K" in
"1") ls -l
uptime;;
"2" pwd;;
"3") echo Bye!;;
*) echo "Choose between 1 and 3!";;
esac
done
User management
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username
passwords
UserID
GroupID
User management
• /etc/passwd : user information (clear text)
• /etc/shadow : passwords (encoded)
• /etc/group : groups
– take a look at these
• groupadd : create groups
• useradd: create users
• /home/username: home of the new users
– May not get created by default
• /etc/default/useradd
• /etc/skel - default directory structure
• Creating a user in linux can be done by modifying a few files
Creating groups
• groupadd gname
• groupadd -g
– add user definied GID (group ID) - usually above
1000
Creating users
• useradd user1
– create user (without settings)
• useradd -D : print the defaults
• useradd -g group1 user3
– primary group
• useradd -g group1 -G gr2,gr3,gr4 user4
– secondary groups
• useradd -m user5
– create home directory automatically
• useradd -p password user6
– create password
Creating users - hardcore way
• add a new line to /etc/passwd
• create the home directory for the new user
• use command passwd username to create a password
• Try the new user!
Additional commands
• passwd
– modify our password
• passwd usernev
– modify the password of an other user
• passwd root - way to "create" root in ubuntu
• groupdel
• userdel
Exercise
• Create two new users. One using commands,
the other by modifying the passwd file
Change ownership
• files have owners
• chown user.group file1
– give your file to someone else
• try it with new new users
– use alt+f2 - f6 to switch between the consoles
Compression
• tar (tape archiever)
• tar -cvzf nameoffile.tar.gz *
– pack and compress everything in cwd
• tar -xvzf nameoffile.tar.gz
– unpack the contents of nameoffile.tar.gz
• Switches
– man tar
– x: eXtract, c: create, v: verbose, z: gzip, f: filename
Exercise 6.5
• Use tar to compress all your shell scripts into
one file
Solution to Exercise 6.5
tar -cvzf myShellScripts.tar.gz *
Exercise 7
• Create a shell script which takes two numbers
as input parameters and add them together
(those of you who already know ifs should write
a full-featured calculator)
Solution to Exercise 7
#!/bin/bash
sum=`expr $1 + $2`
echo "The sum of $1 and $2 is $sum"
Exercise 8
• Write a shell script that adds the current date
(date command) and the current uptime
(uptime command) to the ~/uplog file. There
should be a separator after these two data, so
that the next run can be identified easily.
Solution to Exercise 8
#!/bin/bash
newfile="~/uptime"
date >> $newfile
uptime >>$newfile
echo "----------------" >>$newfile
echo " " >>$newfile
Exercise 9
• Write a shell script that takes two input
parameters from the user, and then creates a
symbolic link pointing to the file denoted by
the first parameter with the name provided as
the second parameter.
Solution to Exercise 9
#!/bin/bash
ln -s $1 $2
Exercise 10
• Write a shell script that prints the contents of
the PATH (the one which holds the names of
the directories where the shell would look for
an executable) variable to a file. The filename
is provided in the first input parameter.
Solution to Exercise 10
#!/bin/bash
echo "Contents of \$PATH: $PATH" > $1
Exercise 11
• Write a shell script that takes the name of a
directory as an input parameter and adds that
directory to the PATH variable. Be careful not
to overwrite the original content, just add it
(and use the standard separator of that
variable).
• Advanced versions (require for and if)
– check if the directory exists
– if the user provides more than one dir, add them
as well
Solution to Exercise 11
#!/bin/bash
PATH="$PATH:$1"
Further exercises
• http://nik.uni-obuda.hu/gwindisch/os_2010/4_1.txt
• http://nik.uni-obuda.hu/gwindisch/os_2010/4_2.txt
• http://nik.uni-obuda.hu/gwindisch/os_2010/4_3.txt
Upload your precious work
• If you want, you can send your work to me. If
the solutions are correct (or interesting), it
could count in your final grades.
• Send it using the commands:
– sudo dhclient eth2 (password: nik119)
– scp name_of_tar hallgato@10.4.1.6:~/
• (password: hallgato)
• Or via email