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บทท�� 4 File Manipulation วิ�บ�ลย์� วิราสิ�ทธิ�ช้�ย์

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OOOOOOOOOO This lecture covers:

• Ways to examine the contents of files.• How to find files when you don't know how thei

r exact location.• Ways of searching files for text patterns.• How to sort files.• Tools for compressing files and making backu

ps.• Accessing floppy disks and other removable m

edia.

OOOOOOOOOO OOOO OOOOOOO 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 app ears to be:

prompt> file myprog.c letter.txt webpage.html myprog.c: C program text

letter.txt: English text webpage.html: HTML document text

Continued

• head, tail filename head and tail display the first and last f

ew lines in a file respectively. You can specify the number of lines as an optio

n, e.g. prompt> head -5 /etc/passwd

prompt> tail -20 /etc/passwd

Continued

• More examples, head sample.f

- display first 10 lines (default)head -5 sample.f - display first 5 lineshead -25 sample.f - display first 25 lines

tail sample.f

- display last 10 lines (default)tail -5 sample.f - display last 5 linestail -5c sample.f - display last 5 characterstail -25 sample.f - display last 25 lines

Continued

• tail includes a useful -f option that can be used to continuously monitor the last few lines of a (possibly changing) file. This ca

n be used to monitor log files, for exampl e:

prompt> tail -f /var/log/messages

continuously outputs the latest additions to the system log file.

Finding Files

• find If you have a rough idea of the directory tree t

he file might be in (or even if you don't and yo u're prepared to wait a while) you can use fin

d:

find directory -name targetfile -print

find will look for a file called targetfile in any p art of the directory tree rooted at directory. ta

rgetfile can include wildcard characters.

Continued

• For example, prompt> find /home -name "*.txt" -print

will search all user directories for any file end ing in ".txt" and output any matching files.

• find can in fact do a lot more than just find fil es by name. It can find files by type (e.g. -typ

e f for files, - type d for directories), by permis sions (e.g. - perm o=r for all files and directori

es that can be read by others), by size (-size ) etc.

Continued

• You can also execute commands on the fil es you find. For example,

prompt> find . -name "*.txt" -exec wc -l '{}' '

;'

counts the number of lines in every text fil e in and below the current directory. The '

{} ' is replaced by the name of each file fo und and the ';' ends the -exec clause.

Continued

• which command If you can execute an application progr

am or system utility by typing its name at the shell prompt, you can use which to find out where it is stored on disk. F or example:

prompt> which ls /bin/ls

Finding Text in Files

• grep (General Regular Expression Print)grep options pattern files grep searches the named files (or standard inp

ut 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:

prompt> grep hello *.txt

searches all text files in the current directory f or lines containing "hello".

Continued

• 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 matchin

g line).• So

prompt> grep -vi hello *.txt

searches all text files in the current directory for lines th at do not contain any form of the word hello (e.g. Hello,

HELLO, or hELlO).

Continued

• If you want to search all files in an entire dire ctory tree for a particular pattern, you can co

mbine grep with find using backward single q uotes to pass the output from find into grep.

• So prompt> grep hello `find . -name "*.txt" -print`

will search all text files in the directory tree r ooted at the current directory for lines contai

ning the word "hello".

Continued

• The patterns that grep uses are actually a speci al type of pattern known as regular expressions

.• Most characters, including all letters and digits,

are regular expressions.• Any other character with special meaning may

be quoted by preceding it with a backslash (\ ).• A list of characters enclosed by '[ ' and '] ' match

es any single character in that list; if the first ch aracter of the list is the caret `^ ', then it match

es any character not in the list.

Continued

• A range of characters can be specified using a dash (- ) between the first and l

ast items in the list.• - - So [0 9] matches any digit and [^0 9]

matches any character that is not a di git.

Continued

• The caret `^ ' and the dollar sign `$ ' are sp ecial characters that match the beginning

and end of a line respectively. The dot '. ' matches any character.

• So prompt> 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.

Continued

• Some complex examples,

prompt> cat hello.txt | grep "dog" | grep -v "cat"

finds all lines in hello.txt that contain t he string "dog" but do not contain the

string "cat".

OOOOOOO OOOOO There are two facilities that are useful for sorting files in

UNIX:• sort filenames

sort sorts lines contained in a group of files alphabetic - ally or (if the n option is specified) numerically. The s

orted output is displayed on the screen, and may be s tored in another file by redirecting the output. So

prompt> sort input1.txt input2.txt > output.txt

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

Continued

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

prompt> sort input.txt | uniq > output.txt

OOOO OOOOOOOOOOO OOO OOOOOO

UNIXsystemsusual l y suppor t a number of ut i l i t i es f or ba cking up and compressing files. The most useful

are:• tar (tape archiver)

tar backs up entire directories and files onto a t ape device or (more commonly) into a single dis

k file known as an archive. An archive is a file that contains other files plus i

nformation about them, such as their filename, owner, timestamps, and access permissions. tar

does not perform any compression by default.

Continued

• To create a disk file tar archive, usetar -cvf archivename filenames

where archivename will usually have a .tar extension. Here the c option mean

s create, v means verbose (output file names as they are archived), and f me

ans file. For example,prompt> tar -cvf play.tar play

Continued

• To list the contents of a tar archive, us e

tar -tvf archivename

For example,prompt> tar -tvf play.tar

• To restore files from a tar archive, usetar -xvf archivename

For example,prompt> tar -xvf play.tar

Continued

• compress, gzip compress and gzip are utilities for compressi

ng and decompressing individual files (which may be or may not be archive files). To compr

ess files, use: compress filename

orgzip filename

In each case, filename will be deleted and rep laced by a compressed file called filename.Z

or filename.gz .

Continued

• For example,prompt> compress play.tar

has a result of play.tar.Zor

prompt> gzip play.tar

has a result of play.tar.gz

Continued

• To reverse the compression process, u se:compress -d filename

orgzip -d filename

• For example,prompt> compress -d play.tar.Z

orprompt> gzip -d play.tar.gz

OOOOOOOO OOOOOOOOO OOOOO UNIX supports tools for accessing removable

media such as CDROMs and floppy disks.

• mount, umount The mount command serves to attach the f

ilesystem found on some device to the files ystem tree. Conversely, the umount comm

and will detach it again (it is very important to remember to do this when removing the

floppy or CDROM).

Continued

• To access a floppy we can use: prompt> mount /mnt/floppy prompt> cd /mnt/floppy prompt> ls

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

prompt> umount /mnt/floppy

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