Files

Everything in UNIX is a "file" -- real files, directories, device drivers, and so forth. This makes it easy to combine files and programs in many different and new ways.

Directories are special files that hold the names of other files or other directories. In this way, the UNIX file system looks like a hierarchical tree, similar to DOS. Note that directories are separated by a forward slash ( / ) not a backslash ( \ ) as in DOS.

File Attributes

file name

A file name is a string of characters. UNIX does not give special relevance to periods or other characters, although some programs expect specific types of filenames.

full path

The full path of a file specifies all directories, such as /usr/local/bin/foo.bar. The filename foo.bar can be used for this file when /usr/local/bin is the current directory.

file permissions

Each file has an associated owner and group. The owner is a logon account, and the group is a possibly empty group of logon accounts. The read, write, and execute permissions (which can be set by the owner) can be different for the owner, group, and public (other). Permissions apply to directories as well as files.

Enter ls -l to find the permissions on the files and subdirectories in the current directory. This returns lines that look something like the following:

 drwx------ 2 bobg comp  512 Jun  7 09:49 mydir
 -rwxr-xr-x 1 bobg comp  321 May 30 14:36 myscript

Each line describes one file. From left to right: the permissions, the number of links, the owner, the group owner, the size in bytes, the date and time of the last modification, and the file's name.

The first character of the permissions tells what kind of "file" it is; d for directory, hyphen (-) for regular file. The remaining nine characters are three triplets. The triplets give the read, write, and execute permissions for that file or directory for that file for, respectively, the file's owner, its group owner, and for the public. The r (read), w (write), and x (execute), indicate the presence of read, write and execute permissions; the hyphen (-) indicates their absence. For directories: r permission allows you do list the files in the directory, w permission allows you to create or remove files from the directory, and x permission allows you to cd to the directory.

Thus, in this example, both files are owned by bobg and have the group comp as group owner. mydir is a subdirectory in which only bobg can read, write, and execute; and myscript is a file in which bobg can read, write, execute, and everyone else, including those in group comp, can read and execute but not write.

The chmod command changes file permissions. For example chmod u+x file adds execute permission to file for the owner (user); use u-x to remove execute permission for the owner. u indicates the file's owner (user), g the owner's group, o the public (other), or a for all three; and r is read permission, w is write permission, and x is execute permission.

File Commands

ls

gives a list of filenames in the current directory; 

mv

rename a file: mv oldfile newfile or mv oldfile newdir

cp

copy a file: cp oldfile newfile or cp oldfile newdir

chmod

change permissions; see "File Attributes" above for examples 

rm

remove a file

cd

change directories

lpr

print a file: lpr -P printer-name file

pwd

"print" working directory; returns the current directory

quota

to see how much disk space you've used

mkdir

create a new directory

rmdir

remove a directory

compress

compress a file into one with a .Z extension; uncompress reverses the process

tar

package a group of files into one file for moving or archiving; also extracts tar files 

Other useful commands

df

show disk space usage for the machine

diff

compare two files;

gzip

gzip myfile compresses myfile . The compressed version of myfile has the same name with an appended extension of .gz and when possible has the same file ownerships, access, and modification times

gunzip

gunzip myfile.ext decompresses (restores to its original form) myfile.ext where ext can be: .gz .taz .tgz .z -z z or .Z . gunzip decompresses files created by: gzip , zip , compress , compress -H , or pack;

logout
exit

end your UNIX session;

man

look up a manual page; man ls will tell about the options to the ls command, for example; main online documentation for UNIX

More

make the output stop after each screenful; Spacebar displays next screenful, Ctrl-b displays the previous screenful, q quits

Passwd

change your password;

Quota

check your own disk space usage and quota

Examples

who am i

To do the obvious; just who lists everyone who is logged on; likewise hostname tells you the name of the machine you're using

pwd

To display current directory

mkdir foo

To make a new directory called foo.

cd

To change to your home directory from anywhere.

cd bin

To change to the bin directory under the current directory.

cd /usr/local/bin

To change to the /usr/local/bin directory, regardless of what your current directory is; much of the public software is stored in the subdirectories of the usr directory

ls -l -a -R | more

To get a list of all files in a directory. -l to get a long listing, with permissions; -a to include files with filenames beginning with a period (.); and -R (note the R is uppercase) to also list the files in all subdirectories of the current directory. And since this is likely to be a long listing, it's "piped" into more so the display pauses at the end of each screen.

ls -l a*.c

To get a long listing of all files with filenames starting with a and ending in .c.

more .profile

To display your .profile file, when your current directory is your home directory.

more ~/.profile

To display your .profile file when your current directory is not your home directory.

cp ../foobar .

To copy the file foobar from the parent of the current directory into the current directory..

rm -i ??

To remove (erase) all files in the current directory with exactly 2 characters in the filename, verifying each erase with a y or n .

mv foobar ../newdir/fubar

To rename the file foobar to fubar and place it in the directory newdir, that is a child of the parent directory of your current directory.

chmod u+x foobar

To make the file foobar executable by its owner.

chmod -R a+r *

To make all files the current directory and all files in all subdirectories (-R) readable by everyone.

man ls

To get information on the command ls and its flags.

uncompress foo.Z

To uncompress the compressed file foo.Z in the current directory.

tar -xf foo.tar

To extract the contents of the file foo.tar in the current directory.