Guide to using the Linux system at the physics department
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Chapter 3. Files, directories and Unix commands

Table of Contents
Unix commands for files and directories
Options in Unix commands
Some files of interest

The Linux file system is a hierarchical file system organized as a tree, files are said to reside in a directory, directories can contain sub-directories.

Each file has a unique absolute filename, e.g. /usr/bin/ssh, /etc/passwd, and /home/einstein/photoeffect.tex. File names may however also be given as relative file names. Such names are relative to the current working directory.

Unix commands for files and directories

Print current working directory
$ pwd /home/einstein
Change directory
cd <directory>
If cd is given with no argument it takes you to your home directory.
List files
ls <file(s)> <directory(s)>
Lists all files in the current working directory if no argument is given.
mkdir make directory
Copy file(s). The simplest use, copy file1 to file2, is obtained with
$ cp file1 file2
but one can also specify a directory as the second argument. It is then possible to give several source files and copy them all at the same time:
$ cp file1 <file2> directory
Note that "." means the current working directory. The final period is therefore essential in commands like
$ cp /home/stud-intro/src/firstprog/* .
The symbol "*" is a kind of wildcard and is discussed in Chapter 4.
The mv command may also be used in two different ways. If the destination is a directory it moves the file(s)
$ mv file1 <file2>... directory
otherwise it renames the file file1 to file2:
$ mv file1 file2
Remove one or several files
$ rm file
A necessary but dangerous command, especially when combined with wildcards.
Type a file to the terminal
$ cat file1 <file2>...
The name comes from concatenate (hoplänka, sammanbinda); since the command may be used to add together several files into a single one.
A pager that lets you page through the file:
$ more file
- SPACE takes you one screen forward.
- "b" takes you backward.
- "q" exits more.
Printing is done with the command lpr. To print file:
$ lpr file
The printer queue is interrogated with lpq.

Options in Unix commands

The default (standard) workings of many commands may be changed by supplementing them with an option. The options usually start with a hypen. Options are often used with the ls command. Try the following ones:

$ ls -a
$ ls -F
$ ls -l

The last example gives a long listing

-rw-r--r--    1 olsson   olsson      23254 Aug 26 09:13 linux.mac
drwxr-xr-x    2 olsson   olsson       4096 Aug 25 14:37 html

The information given here is, in short:

First letter, the kind of file, here ordinary file and directory.
The nine following letters: access rights, r=read, w=write, x=execute. This information appears three times for user, group, and others.
The number of links to this file (don't bother).
Name of the user that owns the file.
Name of the group that owns the file.
File size in bytes.
Date and time for last modification.
File name

Some files of interest

Certain files controls how the shell (Chapter 4), emacs (Chapter 5), and other programs behave. The most important of these are located in your home directory, they all start with a period ('.'), and are therefore not displayed when using ls unless the -a option is used:

$ cd
$ ls -a
.              .alias         .bash_profile  .cshrc
..             .bash_history  .bashrc        .mozilla

Of these, .bashrc contains startup instructions for your shell.

There is also lots of configuration information in the /etc directory. Your personal settings will, however, always take precedence.

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