IT 244: Introduction to Linux/Unix

IT 244: Introduction to Linux/Unix
Class 11

Today's Topics

Reading Assignment

The reading assignment for this week is chapter 5 of Sobell, The Shell.

Homework 6

I have posted homework 6 here.

It is due this coming Sunday at 11:59 PM.

Mid-term

The mid-term exam for this course will be held on Tuesday, October 24th.

It will consist of 25 questions like those on the quizzes.

You will have the entire class period to work on the exam.

60% of the questions will come from the Ungraded Class Quizzes.

The last class before the exam, Thursday, October 19th, will be a review session.

You will only be responsible for the material in the Class Notes for that class on the exam.

The Mid-term is a closed book exam.

Quiz 3

I have posted the answers to Quiz 3 here.

Tips and Examples

Viewing Directory Permissions

If you run ls -l on a directory ...
it will show the permission of everything inside that directory
What if you wanted to see the permission on the directory itself
You have two options
You can run ls -l on the parent directory ...
or you can run ls -ld
The -d option tells ls ...
to display the directory ...
not the things inside the directory
So if I were in a directory that contained another directory named dir
```
$ ls
dir  test
```
and I ran ls -l on dir ...
it would show me the permission on everything inside dir ...

not on dir itself

$ ls -l dir
total 0
-rw-rw-r-- 1 ghoffman faculty 0 Oct  7 21:56 bar.txt
-rw-rw-r-- 1 ghoffman faculty 0 Oct  7 21:56 foo.txt

I could run ls on the parent directory

$ ls -l
total 2
drwxrwxr-x 2 ghoffman faculty 512 Oct  7 21:56 dir
drwxrwxr-x 2 ghoffman faculty 512 Oct  7 22:10 test

or run ls -ld directly on the directory

$ ls -ld dir
drwxrwxr-x 2 ghoffman faculty 512 Oct  7 21:56 dir

Never Use . . in an Absolute Pathname

Sometimes when scoring a homework script I'll come across a pathname like this
```
/courses/it244/f16/ghoffman/../..
```
This is a valid pathname ...
but it's stupid
Since the first character in the pathname is / ...
this is an absolute pathname ...
which means it will work whatever directory you are in
. . means your parent directory
. . should only be used in a relative pathname
When you use it in an absolute pathname ...
it merely goes back to a previous directory
So the pathname above first goes down to /courses/it244/f16/ghoffman
and then goes up two directories to /courses/it244 ...
so it's just a longer way of writing the absolute pathname /courses/it244
You should never write anything on the command line ...
that is longer than you need it to be ...
because that increases the chance of an error

Write Permissions on a Directory

Write permission on a directory only applies to the content of the directory
It does not apply to the directory itself
If you have write permission on a directory you can
- Create a file or directory inside the directory
- Delete a file or directory inside the directory
- Rename a file or directory inside the directory
In order to delete of change the name of the directory itself ...
you must have write permission on the parent directory

Using the `ls` Command

You don't have to be in a directory to run ls on it
If you run ls without an argument ...
it will list the contents of your current directory
But if you give ls a directory pathname ...

it will list the contents of that directory

$ ls ~tsoro/course_files/
cs115_files  it244_files  it341_files

If you want to see the access permissions on the directory it244_files ...
you might be tempted to run ls -l on this directory ...

but if you did you would not get the permissions on it244_files

$ ls -l ~tsoro/course_files/it244_files/
total 57
-rwxrwxr-x 1 ghoffman faculty  163 Sep 15 12:48 bother10.sh
-rwxrwxr-x 1 ghoffman faculty  131 Sep 15 12:48 bother.sh
-rwxrwxr-x 1 ghoffman faculty  167 Sep 15 12:48 break.sh
...

Instead you see the permissions on the contents of it244_files
To see the permissions for the directory itself ...
and not the contents of the directory

you need to use the -d (directory) option along with the -l option

$ ls -ld ~tsoro/course_files/it244_files
drwxrwxr-x 6 ghoffman faculty 1536 Sep 15 12:51 /home/ghoffman/course_files/it244_files

The `chmod` Chant

All of you need to memorize the following

read    write    execute
4       2        1
owner  group    everyone

This is all the information you need to use chomd ...
with the numeric format for specifying permission

Using . When Copying a File

Sometimes, when copying a file to their current directory ...
students will use a long pathname

In other words, they do something like this

$ pwd
/home/it244gh/it244/work
    
$cp ../test.txt ~/it244/work

They could have saved themselves some typing by using .
. stands for your current directory ...
wherever you are in the filesystem
In the above example, . would have made things much easier
```
$cp ../test.txt .
```

Review

The root Account

On every Unix or Linux system there is a special account named root
root can access any file ...
or run any program
root is an administrator account
It is used for system configuration and maintenance
Even a system administrator should not log in as root
Instead he or she should use a regular Unix account ...
and use sudo when running a command ...
that needs root privileges
sudo allows a user to run a command ...
that normally only root can run
When you run sudo it asks you for your password ...
not the password of the root account
In order to run sudo you must be on the sudoers list ...
a change which only the root account can make

Setuid and Setgid Permissions

Sometimes a program needs to read or modify a file ...
to do the work it was designed to do
For example the passwd command ...
which is used to change the password for an account ...
has to make changes to the file /etc/shadow
When you need to change your password you run passwd
But /etc/shadow is owned by the root account ...
and no other account can change it
To deal with situations like this, two special permissions were created
- setuid
- setgid
If a program file has setuid permission ...
anyone who runs the program ...
has all the permissions of the owner of that file ...
but only while the file is running
This permission means it can change any other files ...
that the script or program needs to do its job
If a program file has setgid permission set ...
anyone who runs the program ...
has the permissions of the group assigned to that file...
while the file is running
setuid and setgid permissions are only given to executable files ...
that is, programs and scripts
But while the permission is set on the executable files ...
it works on other files ...
which the executable file has to change
A file with setuid permission ...
will have s in place of x ...
in the column for the owner's execute permission
A file with setgid permission ...
will have s in place of x ...
for the group execute permission
Since setuid and setgid permission ...
apply only to executable files ...
there is no ambiguity in replacing x with s

Directory Access Permissions

The Unix access permissions work a little differently for directories ...
than they do for files
Read and write permissions for a directory are similar to those for a file
Read permission allows you to list the contents of that directory using ls
Write permission allows you to create, delete ...
or change the name of entries in that directory
Write permission on a directory ...
does not allow you to change the contents of a file ...
in that directory
If you have write permission on a directory you can change what's inside it ...
but you cannot rename the directory or delete it ...
unless you have write permission on its parent directory
Execute permission on a directory allows you to do two things
It allows you to enter that directory using cd
It also allows you to read a file in that directory
if you already have read permission on that file ...
or change the contents of a file if you have write permission
Without execute permission on a directory ...
you cannot access any file in that directory

Links

Links are like shortcuts on a Windows machine
or aliases on a Mac
Links allow you to move around the filesystem using short names
Each of you has an entry in your home directory called it244

In the home directory of my test account, it244gh, I have such a link

$ ls -l it244
lrwxrwxrwx 1 ghoffman faculty 36 Oct  5 11:42 it244 -> /courses/it244/sum17/ghoffman/ghoffman

This is a link to/courses/it244/sum17/ghoffman/ghoffman
If you cd into this location and use pwd
```
$ cd it244
    
$ pwd
/home/tsoro/it244
```
This path reflects the route you took to get here
But it is not the real pathname of the directory
You can only get that information if you use pwd with the -P (note the capitalization) option
```
   
$ pwd -P
/courses/it244/sum17/ghoffman/ghoffman
```

The Two Types of Links

There are two types of links
- Hard links
- Symbolic, or soft, links
Hard links are older
A hard link is like a duplicate file name
Hard links can only point to files ...
not directories
You can only have a hard link to a file ...
if that file is on the same hard disk volume ...
as the link
Symbolic links are much more flexible
You can use either an absolute or relative pathname ...
when creating a symbolic link
A symbolic link can point to a file or directory anywhere in the filesystem
Deleting a symbolic link does not delete the file or directory it points to

`ln`

To create a symbolic or soft link, use ln with the -s option

ln takes two arguments, a pathname and the name for the link

$ pwd
/home/it244gh
    
$ ln -s ~ghoffman/examples_it244 examples
    
$ ls -l examples
lrwxrwxrwx 1 it244gh libuuid 28 2012-09-17 17:53 examples -> /home/ghoffman/examples_it244

Removing a Link

To delete a link, use rm
If you delete a symbolic link ...
it will not affect the file or directory it points to

New Material

Syntax of the Command Line

The syntax of the command line is straightforward
First comes the command ...
then, perhaps, some options ...
finally some arguments

The command is executed when you hit the Enter key

COMMAND  [OPTIONS]  [ARG_1]  [ARG_2]  ...  [ARG_N]

The brackets indicate that the contents are optional
Commands vary in the number of arguments they accept
Some accept none ...
others require a specific number of arguments ...
still others accept a variable number of arguments
Arguments must be separated by one or more spaces

Command Options

Many commands have options
Options modify the behavior of the command
Options are usually preceded by one or two dashes, -
GNU programs frequently have options that are preceded by two dashes, --
The options in GNU programs are usually words
The options in other Unix programs are usually a single letter
When a command uses a single dash, -, before an option ...
you can usually combine options ...
behind a single -
An example of this is ls -ltr
This means run ls
- To get a long listing
- Sorted by modification date and time
- In reverse order
Options using two dashes, --, cannot usually be combined
In this case, each option must be written separately ...
and preceded by two dashes
Sometimes the option can have it's own argument
When this happens, the argument is usually separated from the option by spaces
```
gcc  -o  prog  prog.c
```
Utilities that report the size of files usually do so in bytes
This works well with small files
But with large files, a size in bytes can be hard to read
Such utilities often have a -h, or --human-readable, option
With this option, the file size will be displayed ...
in kilobytes, megabytes or gigabytes, as appropriate

df (disk free) shows the amount of space on the various filesystems

$ df 
Filesystem                       1K-blocks     Used Available Use% Mounted on
/dev/sda1                         12253360  6027788   5580096  52% /
none                                     4        0         4   0% /sys/fs/cgroup
udev                               2013316        4   2013312   1% /dev
tmpfs                               404836      560    404276   1% /run
none                                  5120        0      5120   0% /run/lock
none                               2024172        0   2024172   0% /run/shm
none                                102400        0    102400   0% /run/user
blade66:/disk/sd0g/courses/it244   8260768  2615904   5562240  32% /courses/it244
blade82:/disk/sd0f/home/jdu        8260768  8171264      6912 100% /home/jdu
mx1:/disk/sd1e/spool/mail          4129312  3403648    684384  84% /spool/mail
blade82:/disk/sd1h/home/ghoffman  78805984 32988544  45029408  43% /home/ghoffman

When used with the -h opition ...

df produces more readable output

$ df -h
Filesystem                        Size  Used Avail Use% Mounted on
/dev/sda1                          12G  5.8G  5.4G  52% /
none                              4.0K     0  4.0K   0% /sys/fs/cgroup
udev                              2.0G  4.0K  2.0G   1% /dev
tmpfs                             396M  568K  395M   1% /run
none                              5.0M     0  5.0M   0% /run/lock
none                              2.0G     0  2.0G   0% /run/shm
none                              100M     0  100M   0% /run/user
blade66:/disk/sd0g/courses/it244  7.9G  2.5G  5.4G  32% /courses/it244
blade82:/disk/sd0f/home/jdu       7.9G  7.8G  6.8M 100% /home/jdu
mx1:/disk/sd1e/spool/mail         4.0G  3.3G  669M  84% /spool/mail
blade82:/disk/sd1h/home/ghoffman   76G   32G   43G  43% /home/ghoffman

The -h also works with ls
If I run ls -l on a directory ...

the file size will be given in bytes

$ ls -l
total 1236
-rw-rw-r-- 1 ghoffman faculty 26211 Jun 12 09:23 01_class_notes_it244.html
-rw-rw-r-- 1 ghoffman faculty 26138 Jun 21 15:25 02_class_notes_it244.html
-rw-rw-r-- 1 ghoffman faculty 29395 Jun  6 11:21 03_class_notes_it244.html
...

But if I use ls -lh ...

I get a much more readble result

$ ls -lh
total 1.3M
-rw-rw-r-- 1 ghoffman faculty  26K Jun 12 09:23 01_class_notes_it244.html
-rw-rw-r-- 1 ghoffman faculty  26K Jun 21 15:25 02_class_notes_it244.html
-rw-rw-r-- 1 ghoffman faculty  29K Jun  6 11:21 03_class_notes_it244.html

Many commands display a help message when run with the --help option

$ mkdir --help
Usage: mkdir [OPTION]... DIRECTORY...
Create the DIRECTORY(ies), if they do not already exist.

Mandatory arguments to long options are mandatory for short options too.
  -m, --mode=MODE   set file mode (as in chmod), not a=rwx - umask
  -p, --parents     no error if existing, make parent directories as needed
  -v, --verbose     print a message for each created directory
  -Z, --context=CTX  set the SELinux security context of each created
                      directory to CTX
      --help     display this help and exit
      --version  output version information and exit

Report mkdir bugs to bug-coreutils@gnu.org
GNU coreutils home page: <http://www.gnu.org/software/coreutils/>
General help using GNU software: <http://www.gnu.org/gethelp/>
For complete documentation, run: info coreutils 'mkdir invocation'

All GNU utilities accept this option

`tty`

As you type at the command line ...
the characters you type are collected by a program called tty
tty is a device driver that is part of the kernel
It looks at each character as you type ...
and takes appropriate action
Most of the time, tty just places the character in a buffer
A buffer is a space in memory ...
that holds data for later processing
But tty responds differently to special characters
When the character you type is the backspace ...
it erases the previous character from the buffer
When the character is the Control U ...
tty erases the buffer from the current insertion point ...
to the beginning of the line
tty is responsible for all command line editing
When tty sees a newline character ...
it passes the contents of the buffer to the shell
Newline is the character you get from hitting Enter on a windows machine ...
or Return on a Mac

Parsing the Command Line

The shell takes the contents of the buffer ...
and breaks it up into tokens
Tokens are the strings of text separated by spaces or tabs
This action is called parsing
So parsing the act of making a list of all the strings ...
on the command line ...
and throwing away the whitespace
Next, the shell looks for the name of the command
Usually, the command name is the first string on the command line
The command can be specified by a simple filename
```
ls
```
Or by using a pathname to the executable file
```
/bin/ls
```

The PATH System Variable

When you run a program by typing a pathname at the terminal ...
such as /usr/bin/php ...
the shell has no difficulty finding the executable file to run
How can the shell know where to find an executable file ...
if all it gets is the name of the file?
Programs are executable files ...
that can be stored anywhere in the filesystem
So how does the shell find the correct file?
The shell checks a system variable called PATH
PATH contains a list of directories ...
to search for an executable file ...
whose name matches the command typed at the command line
The shell searches each of these directories listed in PATH ...
until it finds an executable file ...
with the name of the command
PATH always has a default value ...
which is created when the system is installed

Here is the default value on our system

$ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin:/usr/lib/oracle/12.1/client64/bin

The absolute pathname of each directory ...
is separated from the next by a colon, :
If the shell reaches the end of the directory listings in PATH ...
without finding the executable file ...
it will print an error message
If the shell finds an executable file ...
but you do not have execute permission ...
it will tell you this in an error message
You can modify the PATH variable in your own Unix environment
We'll see how to do this in a few classes

Running a Program in the Current Directory

For reasons having to do with security ...
you should never put the current directory, . ...
in the PATH list
Then how do you run a program in your current directory?
You can do this using the following construction
```
./PROGRAM_NAME
```
This will always work ...
regardless of the contents of PATH

Here is an example

$ ls -l foo.sh
-rwxrwxr-x 1 ghoffman grad 16 Oct  1 15:49 foo.sh
    
$ ./foo.sh 
Foo to you

Notice that I did not have to run bash to run this script
That's because the script file has execute permission set

Running the Command Entered on the Command Line

When you type a command at the command line ...
the shell has to find the right executable file ...
by looking through the directories listed in PATH
If you have execute permission on the executable file ...
the shell will ask the kernel to start a process for that program
A process is a running program
Only the kernel can create a process ...
so the shell gives the kernel the pathname of the executable file ...
and the kernel does the rest
Each process needs resources to do its job
One of the most important resources is memory
Each process has memory allocated to it ...
that it alone can use
This prevents one program from interfering with another
In addition to memory, the kernel gives the process
various system resources ...
such as pointers to certain files
The kernel also gives the program the list of tokens from the command line
- The name used to call the program
- The options used
- The arguments used
The shell does not check the options or arguments
The shell has no idea what options or arguments ...
are appropriate to a given program
The program has the responsibility ...
to check the options and arguments for correctness
If the program gets the wrong options or arguments ...
it is the responsibility of the program ...
to print an error message and take appropriate action
While the program is running, the shell waits for it to finish
It does this by going into an inactive state ...
known as "sleep"
When the program finishes it must tell the shell that it is done
It does this by sending the shell an exit status
The exit status is an integer that must be 0 or greater
An exit status of 0 means the program finished without error
Any exit status greater than zero indicates an error
A program can issue different exit status values ...
for different types of errors
You can see the exit status of the last program ...

by looking at the value of the system variable ?

$ cat foo
cat: foo: No such file or directory
    
$ echo $?
1

The exit status is 1, meaning the command failed
Notice that I had to put a dollar sign, $ ...
in front of the name of the variable ...
to get its value
When the shell gets an exit status ...
it returns to an active state
It prints a prompt to the screen ...
and it waits for the next command

Today's Topics

Tips and Examples

Review

New Material

Reading Assignment

Homework 6

Mid-term

Quiz 3

Tips and Examples

Viewing Directory Permissions

Never Use . . in an Absolute Pathname

Write Permissions on a Directory

Using the ls Command

The chmod Chant

Using . When Copying a File

Review

The root Account

Setuid and Setgid Permissions

Directory Access Permissions

Links

The Two Types of Links

ln

Removing a Link

New Material

Syntax of the Command Line

Command Options

tty

Parsing the Command Line

The PATH System Variable

Running a Program in the Current Directory

Running the Command Entered on the Command Line

Attendance

Class Quiz

Using the `ls` Command

The `chmod` Chant

`ln`

`tty`