ULI101 Week 3

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  • 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:

# File pathname:

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 pwd to ipc144 (.. means parent directory of pwd)
    cd ../ipc144
    # copy file sample.c (location is relative to parent of pwd) 
    # from joe.doe's home directory to your pwd (. means pwd)
    cp ../joe.doe/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 \ is used, this means \\

' ' 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