Disks and file system

du

• du
• 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 

df

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

Filesystem                                          Size  Used Avail Use% Mounted on
/dev/sda1                                           696M   65M  632M  10% /boot/efi
/dev/sda8                                            96G   46G   46G  51% /
/dev/sda9                                            96G  1.5G   90G   2% /DATA
kita.computing.dcu.ie:/local2/home/student/student  2.0T  1.2T  770G  60% /users/student
kita.computing.dcu.ie:/local2/home/tutors           2.0T  1.2T  770G  60% /users/tutors
tmpfs                                               3.9G  115M  3.8G   3% /dev/shm
tmpfs                                               783M  2.3M  780M   1% /run
...

Filesystem                                          Size  Used Avail Use% Mounted on
/dev/sda1                                            71G   33G   35G  49% /
kita.computing.dcu.ie:/local2/home/student/student  2.0T  1.2T  770G  60% /users/student
kita.computing.dcu.ie:/local2/home/tutors           2.0T  1.2T  770G  60% /users/tutors
tmpfs                                               3.9G   16M  3.9G   1% /dev/shm
tmpfs                                               795M  1.8M  793M   1% /run
udev                                                3.9G     0  3.9G   0% /dev

df to see file system type

df -Th 

• Local disk: A version of ext (Extended file system)
• Shared disk: A version of NFS (Network File System, a distributed file system)

Absolute and relative paths

Relative path of a file

index.html

../index.html

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

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).

Advantages of case sensitivity:

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

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

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

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, 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

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 ' '