Sep 05

Recap

Basic commands

$ cat on the wall
the cat is not on the wall! (what can you infer?)
cd, ls, touch

Escaping characters

Escaping characters

$ touch mickey mouse
creates two files - mickey and mouse.
$ touch mickey\ mouse
creates a single file called "mickey mouse".
$ touch escape\\character
creates a single file called "escape\character".
$ touch escape\\\\character
creates a single file called "escape\\character".
About the only thing you can't escape is "/" which is the UNIX symbol for a directory. So,
$ touch \/l
causes a permission denied error. Why?
Can even create "unbalanced filenames". e.g.,
$ touch \"hello
is perfectly legal.

Learning more about commands

Man(ual)

$ cat -n <filename>
spits out contents of file along with line numbers. "-n" is an option to the "cat" command. How can we know what options are available? Use
$ man cat
to explore the manual pages for the cat command. Press arrow keys to navigate, "q" to quit.
The man pages tell us that
$ cat (without any arguments)
is perfectly legal. What does this do?
$ man man is an eerily self-referential but legal command.

More about files and directories

Some new commands

cp <src> <dest> - copy file(s)
Examples:
- $ cp proga prog1
- $ cp ../dir2/proga .
mv <src> <dest> - move, relocates the file(s) into the specified directory.
- Example:
  - $ mv proga ../dir2
- mv is also used to rename a file leaving the location unaltered.
  - Example: $ mv prog1.c prog2.c
mkdir creates a new directory.
rmdir removes a directory (but only if it doesn't contain any files or other subdirectories).
rm (remove) command erases the file. rm does not typically notify user of the action that has occurred or otherwise warn about what is being attempted (unless your login shell performs some customizations).
rm accepts a list of files to erase, e.g., rm a b c
rm -rf proga forces rm to recursively descend into directory proga (or just delete proga if it is a file) and remove files (check the man pages to see why both the "-r" and the "-f" options are needed). Notice that this gets rid of the limitation of rmdir above.
emacs is a simple to use editor, to create files. e.g., emacs file1 opens up an editor to create a file called file1. Type something, save, and quit.
script <filename> records a transcript of your UNIX session in <filename>. To get out of script mode, type exit.

File Links

Links are filenames that refer to the same file. They are used to reduce redundancy. Changes to one affects the other. Both refer to the same physical file on the disk. Created with the ln command.

Two types of links:

Hard links

A direct pointer to a file. If the original file is moved the link remains valid. A file is accessible as long as any hard link to it remains.

Example:
```
   $ ln johnshomework mattshomework
```
In this command, johnshomework is the linked-to file, and mattshomework is the (hard link) to johnshomework. If you change johnshomework and then do a cat on mattshomework, then the changes will be there in it as well (and vice versa). Unfortunately as we learned the hard and embarassing way in class, this doesn't work with emacs.

Symbolic links

A symbolic link (or symlink or soft link) is a pointer to a file name. If the original file is moved or deleted the link becomes invalid. Deleting the link does not delete the original file. Created with the -s option to ln.

Example:
```
   $ ln -s johnshomework mattshomework
```
Same effect as above command, except that a symbolic (soft) link is created. Works with emacs! :)

Beware of "stale" links. Happens when the linked-to file is removed but the symbolic link still remains.
For the source file (i.e., linked-to file), we have a choice of specifying it using an absolute path name or a relative path name (relative to the link). Former is good when the link is moved (using mv). Latter is good when both link and its destination move together. If only the destination moves, then we get stale links (as above).