ULI101 Week 3

Pathnames

• A pathname is a list of names that will lead to a file.
• Essentially they are directories, but a file name itself is a path as well
• The concept of a pathname relates to every operating system including Unix, Linux, MS-DOS, MS-Windows, Apple-Macintosh, etc.

# Directory pathname:
/home/username/ics124/assignments/

# File pathname:
/home/username/ops224/assignments/assn1.txt

Absolute vs Relative Pathnames

Absolute Pathname

• A pathname that begins from root.
• The pathname begins with a forward slash, for example: /home/someuser/unx122

Relative Pathname

• A pathname that is "relative" to the location of the current or "working" directory.
• For example, if we are in your home directory, issuing the command mkdir uli101 will create the uli101 directory in your home directory.
• Rules:

1. A relative pathname does NOT begin with a slash.

2. A relative pathname can use the following symbols can be used at the beginning:

   • .. parent directory (up one directory level)
   • . current directory

3. Not all relative pathnames begin with ..

4. When using relative pathname, make sure you know your present working directory.

   # Change to another directory branch from parent directory: cd
   ../ipc144
   
   # copy sample.c file from joe.professor's home directory to your current directory:
   cp ../joe.professor/uli101/sample.c .

Relative-to-Home Pathnames

You can specify a pathname as relative-to-home by using a tilde and slash at the start, e.g. ~/uli101/notes.html

The tilde ~ is replaced by your home directory (typically home/your.account) to make the pathname absolute.

You can immediately place a username after the tilde to represent another user’s home directory, for example:

• ~jane.somebody expands to /home/jane.somebody but ~ expands to /home/your_home_dir
• similarly ~uli101 expands to /home/uli101 but ~/uli101 expands to /home/your_home_dir/uli101

Which Type of Pathname to Use?

So far, we have been given many different types of pathnames that we can use for regular files and directories:

• Absolute pathname (starts with / )
• Relative pathname (doesn’t start with /)
• Relative-to-home pathname (starts with ~)

You can decide which pathname type to use to make it more convenient (eg relative - usually less typing or absolute if you don’t know what directory you are currently located in).

Making Directories

Building directories is similar in approach to building a house:

• Begins from a foundation (eg home directory).
• Need to build in proper order (add on addition to house in right location). Use a logical scheme.
• When building directories from different locations, must provide proper absolute or relative pathname.

Planning Directories

Good directory organization requires planning:

• Group information together logically.
• Plan for the future: use dated directories where appropriate (~/christmas/2001, /christmas/2002)
• Too few directories = excessive number of files in each; too many directories = long pathnames.

Where to build directories?

• Want to build a directory called tmp that branches off of your home directory?
• Verify which directory you are located (either look at directory from command prompt or issue the command pwd)
• Type mkdir tmp at the Unix prompt, followed by ENTER
• Optionally you can verify that directory has been created using ls or ls -ld commands)

Creating Parent Directories

By default, a directory cannot be created in a nonexistent location - it needs a parent directory To create directory paths with parent directories that do not exist (using a single command) use the -p option for the mkdir command

# This would create the parent directory mur and then the child directory dir1.
# The -p means "create all the directories in the Path".
$ mkdir -p mur/dir1

Removing Directories

Removing directories is reverse order of building directories

• Issue command rmdir directory
• rmdir cannot remove directories containing files or other subdirectories.
• rmdir cannot remove directories that are anyone's current directory.
• Need to step back to at least parent directory to remove an empty directory.

Removing Sub-trees

• To remove a sub-tree (a directory and all of its contents including sub-directories) use rm -r directory (or rm -R directory).
• The can use the rm -rf command (-f = force) to complete delete files and directories recursiverly, even if they are protected from delete

* Remove files only if you are absolutely sure what you are doing.

• rm -r can erase large numbers of files very quickly. Use with extreme care!
• Backing up your data is a very good idea.

Filename Expansion

• Many of the commands discussed so far make reference to a specific filename - e.g. and regular file to store data, or a directory.
• Sometimes the user may not know the exact name of a file, or the user wants to use a command to apply to a number of files that have a similar name

For example: work.txt, work2.txt, work3.txt

• Special characters can be used to expand a general filename and use them if they match. You may have heard about “Wildcard Characters” - this is a similar concept.
• Symbols:

* (star/asterisk)	Represents zero or more of any characters.
? (question mark)	Represents any single character.
[ ] (character class)	Represents a single character, any of the list inside of the brackets. Placing a ! Symbol after first square bracket means “opposite”). Ranges such as [a-z] or [0-3] are supported.

• To demonstrate filename expansion, let’s assume the following regular files are contained in our current directory:

$ touch work1.txt work2.txt work3.txt work4.c worka.txt working.txt
$ ls
work1.txt work2.txt work3.txt work4.c worka.txt working.txt

• Note the results from using filename expansion:

$ ls work*
work1.txt work2.txt work3.txt work4.c worka.txt working.txt

$ ls work?.txt
work1.txt work2.txt work3.txt worka.txt

$ ls work[1-3].txt
work1.txt work2.txt work3.txt

$ ls work[!1-3].txt worka.txt

UNIX shell

• Command interpreter for UNIX
• Acts as a mediator between user and UNIX kernel
• Processes and/or executes user commands
• More than one command can be executed on one command line when separated by a semi-colon
• You will be learning approx. 30 Unix commands in this course. This is a small, compared to the the 1000+ Unix commands out there
• The term command and utility mean the same in Unix UNIX shell
• There are several kinds of shells available for UNIX
• Most popular shells are:

C shell (this is not the C programming language)

Korn shell - used with Unix

Linux machines most often use the BASH shell (Bourne-Again Shell)

• Each user on one machine can run a different shell
• UNIX scripting = UNIX shell programming

Why command line?

• Why don’t we just use the GUI (KDE, Gnome or some other window manager)? - GUI may not always be available
• What if something is broken?
• What if you are connecting through a terminal remotely? - GUI is for regular users
• Many administrative tools are hard to find in the menus - Command line is more efficient
• Tasks are completed faster
• Less system resources are wasted - Command line allows you to automate repeating tasks through scripting
• Writing scripts requires you to know commands

Command Execution

• While command is being executed the shell waits for it to finish
• This state is called sleep
• When the command finishes executing the shell brings back the prompt
• It is possible to get the command prompt before the command finishes
• This requires executing a process in the background

Command Line Syntax

• A line which includes UNIX commands, program and shell script names and their arguments is called a command line
• Typical command line execution would include:
• Command line parsing
• Breaking it up into tokens
• Executing tokens
• Command line tokens are separated by whitespace
• Command line is actually executed when the Enter key is pressed

Command Editing

• Previously executed commands can be recalled
• The Bourne shell uses the up/down arrow keys to accomplish that
• Other shells may use some other mechanism, for example Korn shell uses vi-style command editing
• Recalled commands can be easily edited before re-executing
• Useful BASH keyboard shortcuts:

Go to the beginning of the line	CTRL+A
Go to the end of the line	CTRL+E
Erase Characters	Backspace or CTRL-Backspace or CTRL-h
Delete a word before the cursor	CTRL-w
Delete everything from to the beginning of line	CTRL-u
Clear Screen	CTRL-l
Search for a keyword in previous commands	CTRL+R
Auto complete file/directory names	Tab

Quoting in UNIX

• Sometimes it may be necessary to use characters that have special meaning to the shell
• In such cases such characters may need to be quoted
• There are several ways of quoting special characters in UNIX, including:

Double quotes (“ “)	quote a group of characters
Single quotes (‘ ‘)	quote a group of characters
Backslash quote (\)	quote the one character immediately following the backslash

• Can prevent variable substitution when the $ character is quoted

# shows all filenames in your pwd
$ echo *

# displays the character *
$ echo \*

• To quote a \ another \\<code> is used

' ' Quotes

• Forward single quote - different than the back tick (backward single quote)
• Quotes all that is inside, preventing wildcard and variable substitution

# shows all hidden files in pwd
echo .*

# displays the two characters '.' and '*'
echo '.*'

Double Quoting

• Commands such as echo can have their arguments quoted using double quotes
• Such quoting can preserve and/or include whitespace
• Variable substitution takes place
• Double quotes do not:
• Prevent shell variable substitution
• Stop escape characters interpretation