1. C151 Multi-User Operating Systems
Linux Commands

2. Expressions in Commands
Current directory: .
Parent directory: ..
Home directory: ~
Root directory: /

3. Path
Absolute path: complete sequence of directories to a file starting from the root directory /: /home/yul/c151/cat.jpg
Relative path: the sequence of directories to reach a file starting from the current directory: c151/cat.jpg
The absolute paths have the same meaning no matter what the working directory is. The relative paths depend on the current working directory.
An absolute path is the concatenation of the current working directory and the relative path: /home/yul/ + c151/cat.jpg

4. Linux File Systems
/      root directory /bin   executable for single user mode /dev   devices that can be mounted by the system /etc  local configuration, administrative things /home home directories for most users /lib   shared libraries /mnt   temporary mount point for mountable devices /proc  kernel and process information /root  home of the superuser (root) /sbin  system binaries (executables) /tmp   temporary files /usr   lots of information used by the system

5. Linux Users
There is one special user that has administrative powers - the superuser. It's name is root. Every user has a home directory that can be found under /home/username. They have an allocated disk space.
Every user’s home directory contains configuration (resource) files that define their environment. They all start with a dot. Example: .signature.
For example, the file ".cshrc" will always be executed whenever the user logs in or opens a terminal window.

6. Environment Variables
The environment variables are global variables that are defined within the shell and that can be used for many purposes.
Their names are in general in all uppercase.
Display the value of an environment variable: echo $PATH
$PATH
lookup list for binaries to be executed.
$HOME
user's home directory

7. Linux Commands
Small utilities/programs under Linux that can take options and arguments.
Argument: what the command applies to ls /usr
Option: how do we want the command to be executed ls –l
The options and arguments can be combined: ls -l /usr/include
Some commands take more than one argument
cp requires a source file and a destination file.

8. More On Commands
Some useful commands: cd, ls, man, pwd, cat, find, grep, wc
Manipulating files and directories: mv, rm, cp, mkdir, rmdir

9. Linux Commands ls: list files cd: switch working directories
Example: ls ~
cd: switch working directories
Example: cd \
pwd: display current working directory
Example: pwd
man: display manual of command
Example: man ls
cat: display the entire content of a file
Example: cat lab1.txt

10. Linux Commands mkdir: create a directory rm:delete files
Example: mkdir C151
rm:delete files
Example: rm *.txt
rmdir: delete a directory
Example: rmdir C151

11. Linux Commands Viewing the Contents of a File The more command
Displays the entire contents of a file one page at a time
Spacebar moves through file one page at a time
Enter key moves through file one line at a time
Only moves forward through a file, not backward
Exit: q
Example 1: more lab1.txt
Example 2: man ls | more

12. Linux Commands Viewing the Contents of a File (continued)
The less command
Nearly equivalent to the more command
Allows you to move forward and backward in the file
Usage: same as more

13. Linux Commands Viewing the Contents of a File (continued)
The head command
Displays just the first ten lines of a file
The tail command
Displays just the last ten lines of a file
Usage: same as more and less

14. Linux Commands find ./ -name hw1.txt find / -name “*.txt”
The find command: find files
The argument is a path. The name of the file must be specified with the option –name.
There more options
Find file with name “hw1.txt” in current directory (including sub directories)
find ./ -name hw1.txt
Find file with name extension “.txt” in root (including all sub directories)
find / -name “*.txt”

15. Linux Commands
The grep command: match a regular expression against text in a file
Display the lines that contain ligyu in file file1.txt
grep “ligyu” file1.txt

16. Linux Commands mv ~/a.pp ./ mv ~/a.pp ~/b.cpp Examples:
mv: rename or move files
Requires two parameters: name of original file and new name or location of file
Examples:
Move file “a.cpp” from home directory to current working directory:
mv ~/a.pp ./
Rename file “a.cpp” in home directory to “b.cpp”
mv ~/a.pp ~/b.cpp

17. Linux Commands cp ~/a.pp ./ cp ~/a.pp ~/b.cpp Examples: cp: copy files
Requires two parameters: path and name of the original file and path and name or the new file
Examples:
Copy file “a.cpp” from home directory to current working directory (without changing name):
cp ~/a.pp ./
copy file “a.cpp” in home directory to “b.cpp”
cp ~/a.pp ~/b.cpp

18. Text Editor: Pico
Provides a series of commands at the bottom of the screen
Backspace, Delete, and other keys work as expected
Easy to Use
Start Pico
Pico file_name
Exit Pico
[ctrl]+x

19. Linux Processes kill pid
A process is any running program. Under Linux a process has an identity (PID), a parent process (PPID), an owner (UID).
To view the active processes: ps Useful options: -A (all), -l (long list)
Stop the execution of a program launched from a terminal: Ctrl-c
Stop the execution of any process
kill pid
The signal 9 is the most powerful.
Kill a process with id 1258
Kill 1258
kill

20. Launching Jobs on Linux
A job is any executable run from the terminal.
The jobs are launched as foreground tasks by default - they will block the terminal until they are done.
To launch a job in the background, use the symbol & at the end of the command line.
A job can be intentionally brought to the foreground with the command fg.

21. Pipe
Pipe (|) enables you to pass the output of one command through the input of another command
ls -l | more
The way this works is that when the shell sees the pipe symbol, it creates a temporary file to store the result of the first command
Then this file becomes the input to second command

22. Input and Output Redirection
Redirecting the input: command < filename Any input will read from that file (must be file).
Redirecting the output. The simple > rewrites the output file, while the double one >> appends to the file (must be file). command > filename command >> filename
Combine input and ouput redirections
Example:
wc < my_text_file.txt > output_file.txt

23. Compressing Files
The most common utilities for compressing and decompressing files on Linux are tar, gzip, and gunzip.
tar allows us to compress several files (directories) into one archive.
Options: cf for compressing, xf for decompressing.
Example: tar –cf C151.tar C151
gzip compresses one file at a time.
Options: none to compress, -d to decompress.
Example: gzip C151.tar
To archive more than one file, most people use a combination of tar and gzip in that order. Those archives have the extension .tar.gz or simply .tgz.

24. Decompressing Files
Suppose we have a compressed file called C151.tar.gz, how could we decompress it?
gzip –d C151.tar.gz (or gunzip C151.tar.gz)
We get C151.tar
tar –xf C151.tar
We get uncompressed files (folders)

25. File Ownership and Permissions
Every file belongs to a particular user, generally the creator of that file. This is the owner of the file. Users may be organized in groups and a group can have special permissions to a file.
A file can be accessible for reading r, writing w, or executing x. To view the content of a directory, the directory should be executable.
Usually the owner has read and write permission to a file, also execute permission if applicable. The root (super user)has read and write permissions for any file.
The command ls -l displays the permissions of a file for the owner, the group, and everyone else (all).
Change permissions: chmod

26. File Ownership and Permissions
Linux file and folder attributes seen with ls -l
Column of 10 characters on left
First character: file (-), directory (d), or link (l)
2nd, 3rd, and 4th characters show permissions of owner
5th, 6th, and 7th characters show permissions of group
8th , 9th, and 10th characters show permissions of all others

27. Managing Files and Directories with Shell Commands
File and Folder Permissions (continued)
Permissions
R = read
W = write
X = execute
- =disabled
Permission mode values
read=4
write=2
execute=1

28. File Ownership and Permissions
Change permissions with chmod
Requires two parameters
Access mode number
File or directory name to change
Example
chmod 644 reports

29. Reading Assignment
Textbook: Chapter 3, Chapter 4, and Chapter 5