Introduction to UNIX / Linux (More)

Some Commands to get started

Explore commands and options by typing "man (command)"
e.g. "man ps"

clear                   Clear screen

Files


ls                      List files
ls -a                   Show "hidden" files (begin with ".")
ls -l                   Detailed
ls -alR                 Recursive

cat (file)              Type file out in command-line window

more (file)             Type file, pause for each screenful
                        enter for new line, space for next page, q to quit

Manipulating files

Human can use GUI file manager. Programs can do anything a human can do, with these commands. Human sometimes types these commands too.


cp                      Copy files
mv                      Move / Rename files
rm                      Remove files
mkdir                   Make directory
rmdir                   Remove directory

See tutorials on manipulating files with cp, mv, rm, etc.

Detach program (&)


(prog) &                Launch a process detached 
                         from command-line (e.g. windowed)
(prog)                  Command-line frozen until prog exits.

           
eog file.jpg &		Launch image viewer Eye of GNOME on file

Web browsers


firefox &              Launch Web browser from command-line
firefox "URL" &
         
google-chrome &

konqueror "URL" &

Firefox multiple windows is confusing on Gnome.
To find other windows: "Activities"
Or Alt-Tab and pause on Firefox and they pop up.

sort


sort 		Sort alphabetically (pipe some stream into sort)
sort -n		Sort numerically
sort -r		Reverse sort 

sort by 5th column:
sort -k 5

old syntax to sort 5th column:
sort +4

grep


grep      Search for a string in a file or files

grep (string) (file)
grep (string) *html 

grep -i (string) (file)  Ignore case
grep -v (string) (file)  Return lines that do NOT match

find

find      Find files by type/date/name, in this dir or below 

find -type d       Find all dirs 
find -mtime -1     Find files modified today

du


du                      Space used by me

du | sort -n		# why -n?

default is k

listing in M  
du -BM

listing in G  
du -BG

top-level only
--max-depth=1

Printing


lp (file)               Print
lpr (file)              Print (on some systems)
lp -Pl128 (file)        Print on printer l128 (L128)

lpq                     See print queue
lprm                    Remove job from queue

cal


cal                     Calendar

cal 8 1752              Calendar for Aug 1752
cal 9 1752              Calendar for Sept 1752
cal 10 1752             Calendar for Oct 1752

which


which (prog)            
 what runs if "prog" is typed
 may return nothing if prog is an alias

which ls


type (prog)
 returns path of prog
 or else shows what prog is alias for

type h
type history


whereis (prog)		Where the binary, source, manual pages are for this prog
whereis perl

Misc


df -h                   Show space on all disks
df -k                   exact kilobytes


w                       Who is logged in
           (see this when you ssh student.computing.dcu.ie)
 
(command) ; (command)   Multiple commands on same line

wget

Command-line HTTP client.



wget -q -O - URL      					  	Download URL, quiet, output to command-line
 
wget -q -O - URL > file.htm 					Output to file

wget -q -O file.htm  URL  					Output to file

wget -q -O - http://site/file.jpg  > file.jpg 		Output JPEG to file (output JPEG to command-line won't work)

wget -q -O file.jpg  http://site/file.jpg   		Output JPEG to file 


 see more on wget

wget appears in this hacking scene from the movie The Social Network (2010).
Perhaps the most realistic hacking scene in the history of movies.

Absolute and relative paths

Relative path of a file

index.html

What file that refers to depends on what directory you are in now.
It looks for index.html in the current directory.

../index.html

is also relative path. It looks for index.html in the parent of the current directory.

Directory before Command Directory after

/users/gdf cd users/ec2 /users/gdf/users/ec2

/users/gdf cd ../ec2 /users/ec2

/users/gdf cd ec2 /users/gdf/ec2

Absolute path of a file

/dir/dir/dir/public_html/index.html

Gives the full "path" from the root down to the file.
Refers to the same file no matter what directory you are in.

Directory before Command Directory after

/users/gdf cd /users/ec2 /users/ec2

Case sensitivity

Case matters in filenames in UNIX (this is why case often matters on Web).

Question: Is case sensitivity a good thing? Or is it a flaw in UNIX?

Advantages of case sensitivity:

More readable code. You know what to expect.
More variables. num and NUM and Num
Set up conventions, so that NUM probably refers to a compile-time-coded constant, num is a real-time-changing variable, etc.
Quicker/simpler searches on strings and changes of strings, since can just search for the literal string.
Better to be case-sensitive for passwords. - Larger space to pick from. Harder to guess. Good to be "unforgiving" for security.

Not much return for such huge disadvantages:

Millions of programmer and user hours lost on case not right.
Millions of failed "404 Not Found" website hits because of wrong case in the URL.

Solutions to "404 Not Found" because of case:

Set up program to handle 404. See My "404 Not Found" Handler
Detach website URLs from underlying (case sensitive) file system.
e.g. Content Management System.

How to see file system type on Linux

See all disks and their file system type:

df -Th

On DCU Linux you will see these:

A version of NFS (Network File System, a distributed file system)
A version of ext (Extended file system)

Filenames and Special Characters

Long file names and multiple periods OK.
e.g. product.4652.suppliers.us.html

DOS/Windows once had the 8.3 format (still part of legacy command-line interaction on Windows).
UNIX had long filenames from an earlier period.
Comparison of file systems

Limits

The Linux ext3 file system
ext3 allows 255 char file names
Linux has a maximum path name limit of 4096 chars.

Part of the source code on my install:
/usr/include/linux/limits.h
shows the following:

#define NAME_MAX         255    /* # chars in a file name */
#define PATH_MAX        4096    /* # chars in a path name including nul */

Avoid these chars

If the command-line is used to address files, it is best to avoid many special characters in filenames.

Avoid these chars in filenames, because they have meaning to the Unix command-line and utilities:

space (separate arguments) # comment < redirection > redirection ` result of a program | pipe & detach process ; separate multiple commands on the same line * wildcard ? wildcard ^ start of line $ end of line / variable value [ pattern matching ] pattern matching \ "quoted" character / should be in pathname, not filename ' string delimiter " string delimiter ! shell history

On many UNIX/Linux distributions (e.g. openSUSE) you can actually put these chars in filenames. But the file may then be hard to work with at the command-line, and naive scripts may crash.
If you just point-and-click your UNIX files (which is allowed too) then you can use many of these characters.

Stick to these chars

If you're going to use the command-line, best to just use these chars in filenames:

0-9 a-z A-Z Use these inside filename only, not at start or end: . - _

Filename
File name characters
UNIX / Win are quite restrictive - lot of bad chars.
Mac can use almost any char.
Though could then get problems on Mac command line.
Can get problem on UNIX when you download or copy in files from another OS with odd chars in file names.
Filenames with spaces are creeping into the UNIX world, but old scripts may fail with them.
Find any file or directory below current dir with spaces in its pathname:
```
$ find . | grep ' '
```

Directory before	Command	Directory after
`/users/gdf`	`cd users/ec2`	`/users/gdf/users/ec2`
`/users/gdf`	`cd ../ec2`	`/users/ec2`
`/users/gdf`	`cd ec2`	`/users/gdf/ec2`