What is 'inode'?

Ans:

When a file system is created, data structures that contain

information about files are created. Each file has an inode

and is identified by an inode number (often "i-number" or

even shorter, "ino") in the file system where it resides.

Inodes store information on files such as user and group

ownership, access mode (read, write, execute permissions)

and type of file. There is a fixed number of inodes, which

indicates the maximum number of files each filesystem can hold.

A file's inode number can be found using the ls -i command,

while the ls -l command will retrieve inode information.

This is description of inode information  which it contain:

* The length of the file in bytes.

* Device ID (this identifies the device containing the

file).

* The User ID of the file's owner.

* The Group ID of the file.

* The file mode, which determines what users can read,

write, and execute the file.

* Timestamps telling when the inode itself was last

modified (ctime, change time), the file content last

modified (mtime, modification time), and last accessed

(atime, access time).

* A reference count telling how many hard links point to

the inode.

* Pointers to the disk blocks that store the file's content

UNIX File and Directory Management

File naming

File names can be any length

Unix is case sensitive. Upper and lower case letters are different. File "A" is different from "a".

Names may include some punctuation: . (period) -(dash), _(underscore), %(percent) and $(dollar sign)

The extension notion is used to some degree. Some common extensions are

• .cpp
• .h
• .o
• .out
• .a
• .java
• .class

A filename prog.1.java is okay although discouraged 

Directory Structure

• Hierarchical or tree-like, similar to the MSDOS, Windows XP, and many other operating systems
• You may have named links to other points in the directory structure, (e.g. two names for the same physical file), so the directory is not purely hierarchical--> you may have, as a result, multiple paths to the same file.
• / is the top level directory (note use of forward slash)
• /directory/subdir/subdir/..../file (MSDOS just tried to be different \\\\)
• If you don't start a file reference with a / then the path is relative to your current directory "working directory"
• Many of the DOS/Windows directory access and wildcard rules were borrowed from Unix.

Examples

ls /var/mail

ls /home/rhodes 

cat /etc/hosts 

ls ~rhodes (~ is a shortcut way to someone's login directory)  

Some common file management commands

These commands are very basic in function. Be sure you know them well  

The Unix server does not know about other printers on the network.  Printers are accessible only if they have their own IP address and can print postscript.  Do not expect to print on your personal printer unless you ftp the file to your machine and print it from notepad or the ftp application directly.

UNIX Commands and their descriptions

Basic UNIX commands

Note: not all of these are actually part of UNIX itself, and you may not find them on all UNIX machines. But they can all be used on turing in essentially the same way, by typing the command and hitting return. Note that some of these commands are different on non-Solaris machines - see SunOS differences.  

If you've made a typo, the easiest thing to do is hit CTRL-u to cancel the whole line. But you can also edit the command line (see the guide to More UNIX).  

UNIX is case-sensitive.

Files

• ls --- lists your files  
  ls -l --- lists your files in 'long format', which contains lots of useful information, e.g. the exact size of the file, who owns the file and who has the right to look at it, and when it was last modified.  
  ls -a --- lists all files, including the ones whose filenames begin in a dot, which you do not always want to see.  
  There are many more options, for example to list files by size, by date, recursively etc.
• more filename --- shows the first part of a file, just as much as will fit on one screen. Just hit the space bar to see more or q to quit. You can use /pattern to search for a pattern.
• emacs filename --- is an editor that lets you create and edit a file. See the emacs page.
• mv filename1 filename2 --- moves a file (i.e. gives it a different name, or moves it into a different directory (see below)
• cp filename1 filename2 --- copies a file
• rm filename --- removes a file. It is wise to use the option rm -i, which will ask you for confirmation before actually deleting anything. You can make this your default by making an alias in your .cshrc file.
• diff filename1 filename2 --- compares files, and shows where they differ
• wc filename --- tells you how many lines, words, and characters there are in a file
• chmod options filename --- lets you change the read, write, and execute permissions on your files. The default is that only you can look at them and change them, but you may sometimes want to change these permissions. For example, chmod o+r filename will make the file readable for everyone, and chmod o-r filename will make it unreadable for others again. Note that for someone to be able to actually look at the file the directories it is in need to be at least executable. See help protection for more details.
• File Compression

• gzip filename --- compresses files, so that they take up much less space. Usually text files compress to about half their original size, but it depends very much on the size of the file and the nature of the contents. There are other tools for this purpose, too (e.g. compress), but gzip usually gives the highest compression rate. Gzip produces files with the ending '.gz' appended to the original filename.
• gunzip filename --- uncompresses files compressed by gzip.
• gzcat filename --- lets you look at a gzipped file without actually having to gunzip it (same as gunzip -c). You can even print it directly, using gzcat filename | lpr

• printing

• lpr filename --- print. Use the -P option to specify the printer name if you want to use a printer other than your default printer. For example, if you want to print double-sided, use 'lpr -Pvalkyr-d', or if you're at CSLI, you may want to use 'lpr -Pcord115-d'. See 'help printers' for more information about printers and their locations.
• lpq --- check out the printer queue, e.g. to get the number needed for removal, or to see how many other files will be printed before yours will come out
• lprm jobnumber --- remove something from the printer queue. You can find the job number by using lpq. Theoretically you also have to specify a printer name, but this isn't necessary as long as you use your default printer in the department.
• genscript --- converts plain text files into postscript for printing, and gives you some options for formatting. Consider making an alias like alias ecop 'genscript -2 -r \!* | lpr -h -Pvalkyr' to print two pages on one piece of paper.
• dvips filename --- print .dvi files (i.e. files produced by LaTeX). You can use dviselect to print only selected pages. See the LaTeX page for more information about how to save paper when printing drafts.

Directories

Directories, like folders on a Macintosh, are used to group files together in a hierarchical structure.

• mkdir dirname --- make a new directory
• cd dirname --- change directory. You basically 'go' to another directory, and you will see the files in that directory when you do 'ls'. You always start out in your 'home directory', and you can get back there by typing 'cd' without arguments. 'cd ..' will get you one level up from your current position. You don't have to walk along step by step - you can make big leaps or avoid walking around by specifying pathnames.
• pwd --- tells you where you currently are.

Finding things

• ff --- find files anywhere on the system. This can be extremely useful if you've forgotten in which directory you put a file, but do remember the name. In fact, if you use ff -p you don't even need the full name, just the beginning. This can also be useful for finding other things on the system, e.g. documentation.
• grep string filename(s) --- looks for the string in the files. This can be useful a lot of purposes, e.g. finding the right file among many, figuring out which is the right version of something, and even doing serious corpus work. grep comes in several varieties (grep, egrep, and fgrep) and has a lot of very flexible options. Check out the man pages if this sounds good to you.

About other people

• w --- tells you who's logged in, and what they're doing. Especially useful: the 'idle' part. This allows you to see whether they're actually sitting there typing away at their keyboards right at the moment.
• who --- tells you who's logged on, and where they're coming from. Useful if you're looking for someone who's actually physically in the same building as you, or in some other particular location.
• finger username --- gives you lots of information about that user, e.g. when they last read their mail and whether they're logged in. Often people put other practical information, such as phone numbers and addresses, in a file called .plan. This information is also displayed by 'finger'.
• last -1 username --- tells you when the user last logged on and off and from where. Without any options, last will give you a list of everyone's logins.
• talk username --- lets you have a (typed) conversation with another user
• write username --- lets you exchange one-line messages with another user
• elm --- lets you send e-mail messages to people around the world (and, of course, read them). It's not the only mailer you can use, but the one we recommend. See the elm page, and find out about the departmental mailing lists(which you can also find in /user/linguistics/helpfile).

About your (electronic) self

• whoami --- returns your username. Sounds useless, but isn't. You may need to find out who it is who forgot to log out somewhere, and make sure *you* have logged out.
• finger & .plan files  
  of course you can finger yourself, too. That can be useful e.g. as a quick check whether you got new mail. Try to create a useful .plan file soon. Look at other people's .plan files for ideas. The file needs to be readable for everyone in order to be visible through 'finger'. Do 'chmod a+r .plan' if necessary. You should realize that this information is accessible from anywhere in the world, not just to other people on turing.
• passwd --- lets you change your password, which you should do regularly (at least once a year). See the LRB guide and/or look at help password.
• ps -u yourusername --- lists your processes. Contains lots of information about them, including the process ID, which you need if you have to kill a process. Normally, when you have been kicked out of a dialin session or have otherwise managed to get yourself disconnected abruptly, this list will contain the processes you need to kill. Those may include the shell (tcsh or whatever you're using), and anything you were running, for example emacs or elm. Be careful not to kill your current shell - the one with the number closer to the one of the ps command you're currently running. But if it happens, don't panic. Just try again :) If you're using an X-display you may have to kill some X processes before you can start them again. These will show only when you use ps -efl, because they're root processes.
• kill PID --- kills (ends) the processes with the ID you gave. This works only for your own processes, of course. Get the ID by using ps. If the process doesn't 'die' properly, use the option -9. But attempt without that option first, because it doesn't give the process a chance to finish possibly important business before dying. You may need to kill processes for example if your modem connection was interrupted and you didn't get logged out properly, which sometimes happens.
• quota -v --- show what your disk quota is (i.e. how much space you have to store files), how much you're actually using, and in case you've exceeded your quota (which you'll be given an automatic warning about by the system) how much time you have left to sort them out (by deleting or gzipping some, or moving them to your own computer).
• du filename --- shows the disk usage of the files and directories in filename (without argument the current directory is used). du -s gives only a total.
• last yourusername --- lists your last logins. Can be a useful memory aid for when you were where, how long you've been working for, and keeping track of your phonebill if you're making a non-local phonecall for dialling in.

Connecting to the outside world

• nn --- allows you to read news. It will first let you read the news local to turing, and then the remote news. If you want to read only the local or remote news, you can use nnl or nnr, respectively. To learn more about nn typenn, then \tty{:man}, then \tty{=.*}, then \tty{Z}, then hit the space bar to step through the manual. Or look at the man page. Or check out the hypertext nn FAQ - probably the easiest and most fun way to go.
• rlogin hostname --- lets you connect to a remote host
• telnet hostname --- also lets you connect to a remote host. Use rlogin whenever possible.
• ftp hostname --- lets you download files from a remote host which is set up as an ftp-server. This is a common method for exchanging academic papers and drafts. If you need to make a paper of yours available in this way, you can (temporarily) put a copy in /user/ftp/pub/TMP. For more permanent solutions, ask Emma. The most important commands within ftp are get for getting files from the remote machine, and put for putting them there (mget andmput let you specify more than one file at once). Sounds straightforward, but be sure not to confuse the two, especially when your physical location doesn't correspond to the direction of the ftp connection you're making. ftp just overwrites files with the same filename. If you're transferring anything other than ASCII text, use binary mode.
• lynx --- lets you browse the web from an ordinary terminal. Of course you can see only the text, not the pictures. You can type any URL as an argument to the G command. When you're doing this from any Stanford host you can leave out the .stanford.edu part of the URL when connecting to Stanford URLs. Type H at any time to learn more about lynx, and Q to exit.

Miscellaneous tools

• webster word --- looks up the word in an electronic version of Webster's dictionary and returns the definition(s)
• date --- shows the current date and time.
• cal --- shows a calendar of the current month. Use e.g., 'cal 10 1995' to get that for October 95, or 'cal 1995' to get the whole year.

CAT : command

The cat command reads one or more files and prints them to standard output. The operator > can be used to combine multiple files into one. The operator >> can be used to append to an existing file.

The syntax for the cat command is:

cat [options] [files]

options:

Examples:

cat file1 

cat file1 file2 > all 

cat file1 >> file2

You can find out more about these commands by looking up their manpages:  

man commandname --- shows you the manual page for the command

Sort command in unix

About sort

Sorts the lines in a text file.

Syntax

sort [-b] [-d] [-f] [-i] [-m] [-M] [-n] [-r] [-u] [+fields] filename [-o outputfile]

Examples

sort -r file.txt - Would sort the file, file.txt in reverse order.