Introduction to UNIX:  Lecture Three

This lecture covers:

• File and directory permissions in more detail and how these can be changed.
• Ways to examine the contents of files.
• How to find files when you don't know how their exact location.
• Ways of searching files for text patterns.
• How to sort files.
• Tools for compressing files and making backups.
• Accessing floppy disks and other removable media.

As we have seen in the previous chapter, every file or directory on a UNIX system has three types of permissions, describing what operations can be performed on it by various categories of users. The permissions are read (r), write (w) and execute (x), and the three categories of users are user/owner (u), group (g) and others (o). Because files and directories are different entities, the interpretation of the permissions assigned to each differs slightly, as shown in Fig 3.1.

File and directory permissions can only be modified by their owners, or by the superuser (root), by using the chmod system utility.

• chmod (change [file or directory] mode)

    $ chmod options files

chmod accepts options in two forms. Firstly, permissions may be specified as a sequence of 3 octal digits (octal is like decimal except that the digit range is 0 to 7 instead of 0 to 9). Each octal digit represents the access permissions for the user/owner, group and others respectively. The mappings of permissions onto their corresponding octal digits is as follows:
 

---	0
--x	1
-w-	2
-wx	3
r--	4
r-x	5
rw-	6
rwx	7

For example the command:

    $ chmod 600 private.txt

sets the permissions on private.txt to rw------- (i.e. only the owner can read and write to the file).

Permissions may be specified symbolically, using the symbols u (user), g (group), o (other), a (all), r (read), w (write), x (execute), + (add permission), - (take away permission) and = (assign permission). For example, the command:

    $ chmod ug=rw,o-rw,a-x *.txt

sets the permissions on all files ending in *.txt to rw-rw---- (i.e. the owner and users in the file's group can read and write to the file, while the general public do not have any sort of access).

chmod also supports a -R option which can be used to recursively modify file permissions, e.g.

    $ chmod -R go+r play

will grant group and other read rights to the directory play and all of the files and directories within play.
 

• chgrp (change group)

    $ chgrp group files

can be used to change the group that a file or directory belongs to. It also supports a -R option.

Besides cat there are several other useful utilities for investigating the contents of files:

• file filename(s)

file analyzes a file's contents for you and reports a high-level description of what type of file it appears to be:

    $ file myprog.c letter.txt webpage.html
    myprog.c:      C program text
    letter.txt:    English text
    webpage.html:  HTML document text

file can identify a wide range of files but sometimes gets understandably confused (e.g. when trying to automatically detect the difference between C++ and Java code).
 

• head, tail filename

head and tail display the first and last few lines in a file respectively. You can specify the number of lines as an option, e.g.

    $ tail -20 messages.txt
    $ head -5 messages.txt

tail includes a useful -f option that can be used to continuously monitor the last few lines of a (possibly changing) file. This can be used to monitor log files, for example:

    $ tail -f /var/log/messages

continuously outputs the latest additions to the system log file.
 

• objdump options binaryfile

objdump can be used to disassemble binary files - that is it can show the machine language instructions which make up compiled application programs and system utilities.
 

• od options filename (octal dump)

od can be used to displays the contents of a binary or text file in a variety of formats, e.g.

    $ cat hello.txt
    hello world
    $ od -c hello.txt
    0000000  h  e  l  l  o     w  o  r  l  d \n
    0000014
    $ od -x hello.txt
    0000000 6865 6c6c 6f20 776f 726c 640a
    0000014

There are also several other useful content inspectors that are non-standard (in terms of availability on UNIX systems) but are nevertheless in widespread use. They are summarised in Fig. 3.2.
 

File type	Typical extension	Content viewer
Portable Document Format	.pdf	acroread
Postscript Document	.ps	ghostview
DVI Document	.dvi	xdvi
JPEG Image	.jpg	xv
GIF Image	.gif	xv
MPEG movie	.mpg	mpeg_play
WAV sound file	.wav	realplayer
HTML document	.html	netscape

Fig 3.2: Other file types and appropriate content viewers.

There are at least three ways to find files when you don't know their exact location:

• find

If you have a rough idea of the directory tree the file might be in (or even if you don't and you're prepared to wait a while) you can use find:

 $ find directory -name targetfile -print 

find will look for a file called targetfile in any part of the directory tree rooted at directory. targetfile can include wildcard characters. For example:

 $ find /home -name "*.txt" -print 2>/dev/null

will search all user directories for any file ending in ".txt" and output any matching files (with a full absolute or relative path). Here the quotes (") are necessary to avoid filename expansion, while the 2>/dev/null suppresses error messages (arising from errors such as not being able to read the contents of directories for which the user does not have the right permissions).

find can in fact do a lot more than just find files by name. It can find files by type (e.g. -type f for files, -type d for directories), by permissions (e.g. -perm o=r for all files and directories that can be read by others), by size (-size) etc. You can also execute commands on the files you find. For example,

 $ find . -name "*.txt" -exec wc -l '{}' ';'

counts the number of lines in every text file in and below the current directory. The '{}' is replaced by the name of each file found and the ';' ends the -exec clause.

For more information about find and its abilities, use man find and/or info find.
 

• which (sometimes also called whence) command

If you can execute an application program or system utility by typing its name at the shell prompt, you can use which to find out where it is stored on disk. For example:

    $ which ls
    /bin/ls
 

• locate string

find can take a long time to execute if you are searching a large filespace (e.g. searching from / downwards). The locate command provides a much faster way of locating all files whose names match a particular search string. For example:

    $ locate ".txt"

will find all filenames in the filesystem that contain ".txt" anywhere in their full paths.

One disadvantage of locate is it stores all filenames on the system in an index that is usually updated only once a day. This means locate will not find files that have been created very recently. It may also report filenames as being present even though the file has just been deleted. Unlike find, locate cannot track down files on the basis of their permissions, size and so on.

• grep (General Regular Expression Print)

    $ grep options pattern files

grep searches the named files (or standard input if no files are named) for lines that match a given pattern. The default behaviour of grep is to print out the matching lines. For example:

    $ grep hello *.txt

searches all text files in the current directory for lines containing "hello". Some of the more useful options that grep provides are:
-c (print a count of the number of lines that match), -i (ignore case), -v (print out the lines that don't match the pattern) and -n (printout the line number before printing the matching line). So

    $ grep -vi hello *.txt

searches all text files in the current directory for lines that do not contain any form of the word hello (e.g. Hello, HELLO, or hELlO).

If you want to search all files in an entire directory tree for a particular pattern, you can combine grep with find using backward single quotes to pass the output from find into grep. So

    $ grep hello `find . -name "*.txt" -print`

will search all text files in the directory tree rooted at the current directory for lines containing the word "hello".

The patterns that grep uses are actually a special type of pattern known as regular expressions. Just like arithemetic expressions, regular expressions are made up of basic subexpressions combined by operators.

The most fundamental expression is a regular expression that matches a single character.  Most characters, including all letters and digits, are regular expressions that match themselves.  Any other character with special meaning may be quoted by preceding it with a backslash (\). A list of characters enclosed by '[' and ']' matches any single character in that list; if the first character of the list is the caret `^', then it matches any character not in the list. A range of characters can be specified using a dash (-) between the first and last items in the list. So [0-9] matches any digit and [^a-z] matches any character that is not a letter.

The caret `^' and the dollar sign `$' are special characters that
match the beginning and end of a line respectively. The dot '.' matches any character. So

    $ grep ^..[l-z]$ hello.txt

matches any line in hello.txt that contains a three character sequence that ends with a lowercase letter from l to z.

egrep (extended grep) is a variant of grep that supports more sophisticated regular expressions. Here two regular expressions may be joined by the operator `|'; the resulting regular expression matches any string matching either subexpression. Brackets '(' and ')' may be used for grouping regular expressions. In addition, a regular expression may be followed by one of several repetition operators:

`?' means the preceding item is optional (matched at most once).
`*'  means the preceding item will be matched zero or more times.
`+' means the preceding item will be matched one or more times.
`{N}'   means the preceding item is matched exactly N times.
`{N,}'  means the preceding item is matched N or more times.
`{N,M}' means the preceding item is matched at least N times, but not more than M times.

For example, if egrep was given the regular expression

    '(^[0-9]{1,5}[a-zA-Z ]+$)|none'

it would match any line that either:

• begins with a number up to five digits long, followed by a sequence of one or more letters or spaces, or
• contains the word none
You can read more about regular expressions on the grep and egrep manual pages.

Note that UNIX systems also usually support another grep variant called fgrep (fixed grep) which simply looks for a fixed string inside a file (but this facility is largely redundant).

There are two facilities that are useful for sorting files in UNIX:

• sort filenames

sort sorts lines contained in a group of files alphabetically (or if the -n option is specified) numerically. The sorted output is displayed on the screen, and may be stored in another file by redirecting the output. So

    $ sort input1.txt input2.txt > output.txt

outputs the sorted concentenation of files input1.txt and input2.txt to the file output.txt.


• uniq filename

uniq removes duplicate adjacent lines from a file. This facility is most useful when combined with sort:

    $ sort input.txt | uniq > output.txt

UNIX systems usually support a number of utilities for backing up and compressing files. The most useful are:

• tar (tape archiver)

tar backs up entire directories and files onto a tape device or (more commonly) into a single disk file known as an archive. An archive is a file that contains other files plus information about them, such as  their filename, owner, timestamps, and access permissions. tar does not perform any compression by default.

To create a disk file tar archive, use

    $ tar -cvf archivenamefilenames

where archivename will usually have a .tar extension. Here the c option means create, v means verbose (output filenames as they are archived), and f means file.To list the contents of a tar archive, use

    $ tar -tvf archivename

To restore files from a tar archive, use

    $ tar -xvf archivename
 

• cpio

cpio is another facility for creating and reading archives. Unlike tar, cpio doesn't automatically archive the contents of directories, so it's common to combine cpio with find when creating an archive:

$ find . -print -depth | cpio -ov -Htar > archivename

This will take all the files in the current directory and the
directories below and place them in an archive called archivename.The -depth option controls the order in which the filenames are produced and is recommended to prevent problems with directory permissions when doing a restore.The -o option creates the archive, the -v option prints the names of the files archived as they are added and the -H option  specifies an archive format type (in this case it creates a tar archive). Another common archive type is crc, a portable format with a checksum for error control.

To list the contents of a cpio archive, use

    $ cpio -tv < archivename

To restore files, use:

    $ cpio -idv < archivename

Here the -d option will create directories as necessary. To force cpio to extract files  on top of files of the same name that already exist (and have the same or later modification time), use the -u option.


• compress, gzip

compress and gzip are utilities for compressing and decompressing individual files (which may be or may not be archive files). To compress files, use:

    $ compress filename
or
    $ gzip filename

In each case, filename will be deleted and replaced by a compressed file called filename.Z or filename.gz. To reverse the compression process, use:

    $ compress -d filename
or
    $ gzip -d filename

UNIX supports tools for accessing removable media such as CDROMs and floppy disks.

• mount, umount

The mount command serves to attach the filesystem found on some device to the filesystem tree. Conversely, the umount command will detach it again (it is very important to remember to do this when removing the floppy or CDROM). The file /etc/fstab contains a list of devices and the points at which they will be attached to the main filesystem:

$ cat /etc/fstab
/dev/fd0   /mnt/floppy  auto    rw,user,noauto  0 0
/dev/hdc   /mnt/cdrom   iso9660 ro,user,noauto  0 0

In this case, the mount point for the floppy drive is /mnt/floppy and the mount point for the CDROM is /mnt/cdrom. To access a floppy we can use:

$ mount /mnt/floppy
$ cd /mnt/floppy
$ ls (etc...)

To force all changed data to be written back to the floppy and to detach the floppy disk from the filesystem, we use:

$ umount /mnt/floppy
 

• mtools

If they are installed, the (non-standard) mtools utilities provide a convenient way of accessing DOS-formatted floppies without having to mount and unmount filesystems. You can use DOS-type commands like "mdir a:", "mcopy a:*.* .", "mformat a:", etc. (see the mtools manual pages for more details).

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