- UNIX lets you store "stuff" inside files.
- "Stuff" can be a wide variety of things:
- email
- things downloaded off the network
- letters and memos
- spreadsheets, databases, etc.
- configuration information for programs you run
- program source code
- executable code generated by a compiler
- audio files
- ... And so on.
- Standard example of how to think of a file is a normal office folder.
- UNIX also provides directories to let you manage files.
- Usually make a directory to keep related things in, e.g. you might:
- Keep all your email saved in a directory named Mail.
- keep all files related to a certain project in a directory with that project's name.
- Keep all files downloaded off the net in a directory named download.
- Make a directory named tmp for temporary files.
- Keep all your own programs in a directory named bin.
- When sys-admin creates a user account, each user is given their own home directory.
- When you log in, you will be in your home directory.
- The command pwd will show you what directory you are in. It stands for 'present working directory'.
% pwd
/home/visitors/kns
%
- You can move to a different directory with the cd command, but you will need somewhere to go first.
- You can see what files and directories are in your current directory with the ls command.
- Create a directory with the mkdir (make directory) command:
% pwd
/home/visitors/kns
% ls
% mkdir tmp
% ls
tmp
% cd tmp
% pwd
/home/visitors/kns/tmp
%
- Beginners sometimes find themselves lost after using cd to move around, losing track of where they are.
- Always keep the pwd command in mind.
- Using cd with no "arguments" will always return you to your home directory:
% pwd
/home/visitors/kns/tmp
% cd
% pwd
/home/visitors/kns
%
- The UNIX spec provides many more commands to work with files and directories.
- The command to remove directories is rmdir (remove directory):
% ls
tmp
% rmdir tmp
% ls
%
- This only works if the directory is empty. If there are any files or other directories inside of tmp then rmdir will not work.
- As with most UNIX commands, if you have more than one directory to remove, you can give more than one name in the same rmdir command line.
- UNIX organizes files and directories in an upside-down tree structure.
- The top of the "filesystem" is called the root.
- The root is written as "/"
- The root is often verbally called "slash"
- Directories make up the "branches" of the tree.
- Files make up the "leaves".
- Refer to files and directories with "pathnames".
- If a name begins with "/" it is a "full pathname" or "fully-qualified pathname".
- When UNIX searches for the file, it begins at the root.
- In full pathnames, directory names are separated by "/" characters.
- The pwd command above showed a full pathname.
- /home/visitors/kns is the full pathname to my home directory.
- If a name does not begin with "/", it is a relative pathname. When UNIX searches for the file, it begins at the directory you are currently in.
- At any given time, the directory you are in is called your current working directory.
- You will usually use relative pathnames, working with files in your home directory or directories you create.
- The previous example of making and then removing the directory tmp
used a relative pathname.
- Several commands are available to help you manage files.
- The cp command copies files:
% ls
% cp /etc/motd motd
% ls
motd
%
- The mv command moves files:
% ls
motd
% mv motd message-of-the-day
% ls
message-of-the-day
%
- Both cp and mv expect the first argument to be the "source" to copy or move from and the last argument to be the "target" to copy or move to.
- If the last argument is a directory name, the file will be named the same thing but in that directory.
- In this case, you can have more than one source argument (you can cp or mv more than one file at a time if the target is a directory)
% ls
message-of-the-day
% mkdir tmp
% ls
message-of-the-day tmp
% cp /etc/motd message-of-the-day tmp
% ls
message-of-the-day tmp
% ls tmp
message-of-the-day motd
%
- rm command removes files:
% ls
message-of-the-day tmp
% rm message-of-the-day
% ls
tmp
%
- You need to use rmdir on directories, and they must be empty:
% ls
tmp
% rmdir tmp
rmdir: directory "tmp": Directory not empty
% ls tmp
message-of-the-day motd
% rm tmp/message-of-the-day tmp/motd
% rmdir tmp
%
- You can look at what is inside a file with the cat command.
- cat will send the whole file to your screen without stopping
between screenfuls. cat winds up being more useful for other things.
- A better command for viewing files is more. It will stop
after one screenful with a prompt that looks like this:
--More--(81%)
- Here you can type:
- [space character] to see another screenful
- [carriage return] to see one more line
- "b" to go [b]ack one page for most versions of more
- "q" to [q]uit showing you the file
- The number is the percentage of the file you have seen so far.
- Another command many sites have that does the same thing is less. But less is not available in all versions of UNIX.
- There are two special directory names:
- The directory you are currently in is called "." (pronounced "dot").
- The directory immediately above is called ".." (pronounced "dot dot").
- To illustrate:
% mkdir tmp
% pwd
/home/visitors/kns
% cd .
% pwd
/home/visitors/kns
% cd tmp
% pwd
/home/visitors/kns/tmp
% cd .
% pwd
/home/visitors/kns/tmp
% cd ..
% pwd
/home/visitors/kns
%
- use l switch to ls command to show most of the
user-level information UNIX keeps about a file
% ls -l
total 2
-rw-r--r-- 1 kns visitors 1203 Mar 1 23:49 motd
%
- the name of the file appears on the far right
- the date and time is when the file was last modified
- 1203 is the size of the file in bytes
- kns is the username of the file's owner
- visitors is the group the file is in
- the 1 in front of kns is the link count, that
will get covered later
- the group of ten characters at the beginning of the line is the mode
of the file
- the first character indicates the file's type
- "-" means it is a plain file
- "d" means it is a directory
- "p" means it is a pipe
- "l" means it is a symbolic link
- "b" means it is a block special file
- "c" means it is a character special file
- the other nine characters indicate permission settings
- UNIX has three basic operations it will check for a file
- read : contents of the file can be read (looked at with
cat, more, copied with cp)
- write : contents of the file can be written
- execute : file can be used as a command
- UNIX also has three classes of users it will check these
permissions for
- owner of the file
- users in the same group as the file
- anyone else
- in the mode "-rw-r--r--" above:
- first dash means it is a plain file
- next three characters are read/write/execute permissions
for the owner of the file ("-" in one of those positions
means that permission is denied, so the owner can not
execute this file)
- next three characters are read/write/execute permissions
for users in the same group as the file
- last three characters are read/write/execute permissions for
everyone else
- with the above mode
- the owner of the file may read from or write to the file but
not execute it as a command
- users in the visitors group may read the file but not
write to the file or execute it as a command
- the same for anyone else
- the command to change permissions is chmod
- chmod lets you grant or deny permissions to each class of user
- the simplest forms of chmod are:
chmod c+p file [file ...]
chmod c-p file [file ...]
- the first form will grant (add) permissions, the second form will deny
(subtract) permissions
- for both forms c specifies the class
- "u" for user (your) permissions
- "g" for group permissions
- "o" for "other" permissions (everyone else)
- "a" for effecting all three classes at the same time
- the p specifies the permission(s) to add or subtract
- "r" for read permission
- "w" for write permission
- "x" for execute permission
- can give more than one permission here
- replace file with the name of the file you want to change the
permissions on
- [file ...] is a standard way of saying more filename(s) may be
given
- first example denies "other" read permissions:
% ls -l motd
-rw-r--r-- 1 kns visitors 1203 Mar 1 23:49 motd
% chmod o-r motd
% ls -l motd
-rw-r----- 1 kns visitors 1203 Mar 1 23:49 motd
%
- to grant group write permission:
% chmod g+w motd
% ls -l motd
-rw-rw---- 1 kns visitors 1203 Mar 1 23:49 motd
%
- to grant "other" read and write permission:
% chmod o+rw motd
% ls -l motd
-rw-rw-rw- 1 kns visitors 1203 Mar 1 23:49 motd
%
- to deny everyone write permission:
% chmod a-w motd
% ls -l motd
-r--r--r-- 1 kns visitors 1203 Mar 1 23:49 motd
%
- it can be useful at times to deny yourself write permission
- permissions often get represented as octal digits
- here the three groups of permissions (the characters rwxrwxrwx) get
converted to three numbers, each number representing the permissions for
one class
- under this scheme "r" is worth 4, "w" is worth 2, and "x" is worth 1
- add up the numbers representing the letters to get the number for
this octal representation
- permissions of "rw-r-----" would be 640
- permissions of "r--r--r--" would be 444
- permissions of "rwxrw-r--" would be 764
- chmod will take an octal number for a mode setting as well
% ls -l motd
-r-------- 1 kns visitors 1203 Mar 1 23:49 motd
% chmod 644 motd
% ls -l motd
-rw-r--r-- 1 kns visitors 1203 Mar 1 23:49 motd
%
- unless you know octal number schemes fairly well it is best to stick
with the other method of using chmod
- permissions have slightly different meanings for directories
- read permission on a directory means you can get a listing of the
files with ls
- write permission on a directory means you may make new files there
or remove files in the directory
- execute permission means you can access files inside the directory
but only if you know their name, you can't get a listing with ls
- usually unless you are in a special group for sharing information
you will set group permissions to be the same as "other" for your files
and directories
- sys-admin picks a group to put you in when your account is made
- can also add you to "supplemental" groups if necessary for you to
share files with others
- command groups will show you what groups you are in
% groups
visitors lab
%
- here primary group is "visitors" and supplemental group is "inven"
- command chgrp will change a file to a different group
% ls -l motd
-rw-r--r-- 1 kns visitors 1203 Mar 1 23:49 motd
% chgrp lab motd
% ls -l motd
-rw-r--r-- 1 kns lab 1203 Mar 1 23:49 motd
%
- now permissions could be set so only people in group "lab" may access
"motd"
- sys-admin must set up groups
- quickly mentioned modification date of file earlier
- for some things modification time of file is very important
- when using mv modification date will not be changed
- by default when using cp modification date will be the
present time for the target file, source file will be unchanged
- can have cp set modification date of target to be the same
as the source with "p" option
% date
Tue Mar 7 23:26:03 EST 2000
% ls -l motd
-rw-r--r-- 1 kns lab 1203 Mar 1 23:49 motd
% cp motd motd_1
% ls -l
total 8
-rw-r--r-- 1 kns lab 1203 Mar 1 23:49 motd
-rw------- 1 kns visitors 1203 Mar 7 23:26 motd_1
% cp -p motd motd_2
% ls -l
total 12
-rw-r--r-- 1 kns lab 1203 Mar 1 23:49 motd
-rw------- 1 kns visitors 1203 Mar 7 23:26 motd_1
-rw-r--r-- 1 kns lab 1203 Mar 1 23:49 motd_2
%
- touch command will set last modification time to the
current time if file exists, will create the file if it does not exist
- file created by touch will be zero length
% date
Tue Mar 7 23:29:11 EST 2000
% ls -l
total 4
-rw-r--r-- 1 kns lab 1203 Mar 1 23:49 motd
% touch motd foo
% ls -l
total 4
-rw------- 1 kns visitors 0 Mar 7 23:29 foo
-rw-r--r-- 1 kns lab 1203 Mar 7 23:29 motd
%
