Miscellaneous utilities
Although the structure of files on a UNIX machine is
uncomplicated, other operating systems may impose a more complex
structure on how their files are represented. If you need to
convert a file, either to export it to, or to import it from, a
non-UNIX system, use dd ('disk-to-disk'). For example,
some systems require that files be structured as having a sequence
of fixed-size blocks, or might use a
different character set to ASCII. This command can also perform
simple translation of characters - for instance, if you received a
file funny, which contains only upper-case letters,
then dd can create a file with lower-case letters in
place of the upper-case ones:
$ dd conv=lcase <
funny
You can try this. More seriously, if you do need to read from or
write to a file to be used on a non-UNIX system, you should examine
the manual page for dd carefully.
Suppose you wished to run a utility with the arguments to that
utility piped to it. This might be the case if the arguments were
to be split over several lines. A simple example might be if you
had a file list containing filenames, and you wished
to ls -l each of them. Using the mechanisms so far
discussed, the resultant script would be inelegant:
$ X=$( cat list ); eval ls -l
$X
By use of the $( ... ) mechanism, we have
concatenated all the lines of list into a single
string, and have passed that string to ls -l. The
utility xargs can help: it takes as its arguments a
command, and then reading from its standard input appends options
to that command, and then runs it. The above example would then
become:
$ xargs ls -l <list
A more serious, and frequently quoted example of the use of
xargs is in conjunction with find, where
with the -exec argument find might create
a large number of processes. Suppose your home directory contains a
large number of subdirectories, and you wished to perform
ls -ld on each of them. One possibility would
be:
$ find ~ -type d -exec ls -ld {}
\;
but this would create as many processes as directories - it is
inefficient. More effective - and quicker - would be:
$ find ~ -type d -print | xargs ls
-ld
The final three commands that are mentioned in this chapter are
introduced for completeness; they are included in the POSIX
standard, and you should know of their existence.
There is a command logger ('log error message')
which can be used to save a message for reading later on by the
system administrator. It might be used (say) to inform the
administrator if a batch job failed to read a system file
correctly; the user would not be in a position easily to forward
the message, and the action to be taken would definitely be for the
administrator to perform. Unless your administrator has given you
specific instructions on how it should be used, you are probably
advised not to use logger.
Although you are likely to be using UNIX where English is a
normal medium of communication, the concept of
locale is supported whereby both messages from
commands and the character set used can be customised to other
languages. The command locale allows you to examine
the current locale, and localedef to define a new
locale.
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