Unix Tutorial 2

Tree structure of the Unix Filesystem

all the stored information on a Unix computer is kept in a filesystem

the place in the filesystem tree where you are located is called the current working directory

some typical directories on a Unix file system:

/ - root directory, base of the filesystem's tree structure
/bin - command binaries directory, includes executable public programs that are part of the Unix operating system and its utilities
/dev - device directory, contains special files for ttys, ptys, tape drives, and so on
/lib - contains library files for the C and other programming languages
/lost+found - lost files directory, disk errors or incorrect system shutdown may cause files to become "lost"
/usr - contains subdirectories for spooling, mail, locally generated programs, executables for user commands, and other parts of the Unix system
/usr/bin - command binary files and shell scripts directory, similar to /bin
/usr/man - manual pages directory, contains the online version of the Unix reference manuals
/usr/ucb - contains standard Unix commands originally developed at the University of California, Berkeley

Some basic Unix commands for manipulating files

Listing files in a directory

ls -l : long listing
ls -a or ls -A : shows all files including the hidden files
ls -F : shows the type of file by printing an extra character after each filename

(nothing) - regular file
* - an executable
/ - a directory
@ - a symbolic link
= - socket

ls -R : lists all subdirectories, recursively
For example : ls -aF; ls -al

Moving around the file system

cd ~ : goes to the home directory
pushd : adds your directory to the directory stack and moves you to the new directory
popd : takes the top directory off of the directory stack and moves you to the directory underneath
For example:

pwd
pushd ~/pos
pushd
pushd
podd

File Access Permissions

when trying to access a file, you are either the file's owner, a member of the file's group, or an "other"
3 bits determine whether you are allowed to read, write or execute the file
a total of 9 mode bits to set the basic access permissions
For example:

-rw------- is mode 600
-rwxrwxrwx is mode 777
-rw-rw-rw- is mode 666

chmod nnn filename : changes the file access permission of filename according to nnn

Logging In and Logging Out

The 'dot' files in C shell

.login file is read when you start a login shell
.cshrc file is read any time a C shell starts
.logout file is read when you end a login shell

The login process

the login program starts a shell, e.g. the C shell
C shell reads .cshrc in the home directory

shell variables like cdpath, prompt and aliases should be set here
other commands that should run every time you start a shell

C shell (as the login shell) will then read the .login file next

search path, terminal type and environment variables are set here
other commands that you want to run whenever you log in

Shell and Environment variables

Environment variables

the shell need information like what kind of terminal you're using, where any commands you want to use, what is your favourite editor, and so on
Unix uses environment variables to store the above information
For example, the TERM environment variable tells programs what kind of terminal you're using. Otherwise, you might have to type commands like:

mail -editor vi -term aardvark48

environment variables are "inherited" by any program you start, including another shell
Related commands :

setenv NAME value (e.g. setenv EDITOR /usr/bin/vi)
printenv
echo $NAME (e.g. echo $TERM)

Shell variables

shell variables are local to a particular instance of the shell
environment variables are "global" variables, while shell variables are "local" variables
shell variables have lowercase names
Related commands:

set name=value (e.g. set shell=/bin/csh)
set
echo $name (e.g. echo $shell)

More Unix commands

Aliases

"alias" facility lets you define abbreviations for commonly used commands
for example:

alias ll ls -al

alias is stored in .cshrc file

History

the history mechanism allows you to easily recall and repeat past commands
to set C shell history, add "set history=n" in .cshrc file, where n=number of past commands you want to save
related commands:

history (e.g. history 10 will show the last 10 commands)
!! - repeats last command
!so - repeats last command that starts with so
!?fn? - repeats last command that has fn anywhere in it
!12 - executes command number 12
!:0 - selects only the command name; rather than the entire command line
^xy^yx - shorthand substitution command
!!:s/xx/ab/ - another substitution command

Searching Text

grep - to look for strings matching a regular expression and print only the lines found
use grep when you want to look at how a particular word is used in one or more files
options commonly used:

-i : ignore distinction between upper & lower case
-c : return only a count of the number of lines matched
-w : searches for the pattern "as a word"
-l : return only the name of file when grep finds a match
-v : only prints out lines that don't match the search pattern

On-line help

which - tells you which version of the program you are using
where, whereis - locate the executable file, source code and manual pages for a program
man - basis for Unix's online documentation

commonly used sections of the manual:

1 : user commands
2 : system calls
3 : library routines

manual pages:

NAME : the program's name
SYNOPSIS : how to invoke the program
DESCRIPTION : description of what the program does
OPTIONS : an explanation of each option
EXAMPLE : examples of how to use the program
ENVIRONMENT : environment variables that control the program's behaviour