Too small to keep, too big to throw back
Small fry Unix commands, part 3: du, tr, nl
This is the third installment in a series on small-but-useful Unix commands. Mo covers the
du(summarize disk usage),
tr(translate characters), and
nl(line numbering filter) commands. (1,900 words)
If the directory only contains files, it's easy enough to issue an
command and get the sizes of files in bytes and blocks.
$ ls -ls total 6 2 -rw-r--r-- 1 mjb group 3 Feb 04 23:31 minutes.txt 4 -rw-r--r-- 1 mjb group 1201 Feb 04 23:25 note.txt
The first column contains the size of the file in 512-byte blocks, and the sixth column gives the size of the file in bytes. Files in this directory consume 6 blocks, containing only 1204 bytes. In the March column, I discussed allocation units -- the minimum space allocated by the operating system for a file. You should review that article for more details, but here's a brief explanation of how allocation units work.
This method is used in all major operating systems in one form or another. Some convenient number of bytes is selected as the minimum amount that can be allocated to a file. This amount is an allocation unit. If the file doesn't use all the space in an allocation unit, it's recorded at the beginning of the unit, with the remaining space set aside to accommodate further expansion of that file.
As you add to the file, the new data is stored in the empty reserved space on the disk, so long as it doesn't exceed the number of bytes permitted in an allocation unit. Once the file has used all available space, another allocation unit is grabbed and reserved. Any spillover from the first allocation unit is tucked in at the start of second allocation unit, and so on.
Earlier Unix systems used an
allocation unit of 512 bytes. These 512 bytes came to be known as a block. As
disk sizes grew, the basic allocation unit was increased to 1024 bytes on most
systems (larger on some), but many utilities, such as
ls above, still report
file sizes or disk use in 512 byte blocks. So, the 3-byte file uses 2 blocks.
In the following example, the directory in question includes a subdirectory, perl. The 2 blocks allocated for the perl directory are the blocks used only by the directory itself, not those used by the files in the directory.
$ ls -ls total 6 2 -rw-r--r-- 1 mjb group 3 Feb 04 23:31 minutes.txt 4 -rw-r--r-- 1 mjb group 1201 Feb 04 23:25 note.txt 2 drwxr-xrx 2 mjb group 128 Jan 29 18:53 perl
We could figure out the sizes, by doing an
ls -ls perl, but suppose there's
another directory under perl? And what if there were a third directory beneath that one?
How do you du?
The solution to this dilemma is the Unix utility
du. This little utility will
recurse through all subdirectories and display all the blocks being used. In the display below,
the directory being processed contains a perl
subdirectory, which in turn contains a src subdirectory. The src directory
contains files totaling 1540 blocks. The perl directory count includes all the
blocks in src plus the blocks used by files in perl. Finally, the top level
includes all blocks below it, plus blocks used by files used in the current
$ du 1540 ./perl/src 5648 ./perl 5654 .
-a option displays the details for each file.
$ du -a 1500 ./perl/src/big.prl 40 ./perl/src/prog.prl 1540 ./perl/src 4108 ./perl/perl.tar 5648 ./perl 2 ./minutes.txt 4 ./note.txt 5654 .
du command will cut through a lot of
It provides size information as well as a reasonable display of the directory tree.
Switching things around with tr
tr utility translates one set of characters into another.
tr abc def test.txt will process the records from test.txt and will translate the letter a to d, the letter b to e and the letter c to f. At first glance this doesn't seem very useful, unless you want to
practice amateur cryptography, but
tr has additional options that make it much
more powerful. Two examples should give you a feel for the command.
The characters to be translated can be expressed as a range. In the
command below, a directory is output through
translates a to A, b to B and so on -- converting everything from lowercase to uppercase.
$ ls -ls|tr [a-z] [A-Z] TOTAL 6 2 -RW-R--R-- 1 MJB GROUP 3 FEB 04 23:31 MINUTES.TXT 4 -RW-R--R-- 1 MJB GROUP 1201 FEB 04 23:25 NOTE.TXT 2 DRWXR-XRX 2 MJB GROUP 128 JAN 29 18:53 PERL
Using tr in the real world
Among other things, case conversion solves a problem created by some utilities that copy MS-DOS files onto a system. They copy the files using the uppercase convention of MS-DOS, and the file names need to be converted to lowercase to work correctly. Assuming a directory full of files named in uppercase, the following command will rename all the files to lowercase versions. The command takes each file name and echoes it through a pipe using
tr to change
uppercase to lowercase. The result is used as the target of a
$ for name in * > do > mv $name `echo $name|tr [A-Z] [a-z]` > done $
tr includes the
-s switch, which squeezes repeating instances of the
output characters to one instance.
In the following example, the file test.txt contains one line with several
spaces between the words. The
tr command translates each
space into another space, but the
-s option compacts multiple spaces into a single
output space. The resulting file, test2.txt, has a single space between each word.
$ type test.txt How are you today? $ tr -s " " " " < test.txt >test2.txt $ type test2.txt How are you today?
Fancy line numbering with nl
nl utility adds line numbers to a file. Although this would seem like a
nl has a great number of options. To illustrate some of these
options, we're going to undertake the old-fashioned task of adding line
numbers to a Cobol program. I chose this example because it's a great way of illustrating many of the features of
nl. The following listing is
hello.txt, a Cobol program with missing line numbers.
$ type hello.txt IDENTIFICATION DIVISION. PROGRAM-ID. HELLO. ENVIRONMENT DIVISION. DATA DIVISION. PROCEDURE DIVISION. PROGRAM-BEGIN. DISPLAY "Hello world." PROGRAM-DONE. STOP RUN.
The first pass at this is simply to add line numbers, as in the following listing. The output has several problems.
The numbers in this listing start at one and rise in increments of one.
Cobol usually operates in increments of 10 or 100, although one is valid.
Cobol numbering also includes leading zeroes, which this listing doesn't
display. Blank lines should be numbered but aren't. Finally,
default behavior is to add a tab separator after the
number and before the original line. Though the tabs are not visible in this
listing, many Cobol compilers can't handle them at all.
hello.cbl type hello.cbl 1 IDENTIFICATION DIVISION. 2 PROGRAM-ID. HELLO. 3 ENVIRONMENT DIVISION. 4 DATA DIVISION. 5 PROCEDURE DIVISION. 6 PROGRAM-BEGIN. 7 DISPLAY "Hello world." 8 PROGRAM-DONE. 9 STOP RUN.
Let's tackle these problems one at a time. The separator character
can be specified as an ordinary space using the
-s switch (as in
-s" "). The first modified version of the command is shown below.
$ nl -s" "
The format for the number itself is controlled by several options. The
option specifies the width of the number. For Cobol, this width is six.
The default for
nl happens to be six, but I'll include the option to be thorough. The
-v option lets you specify the starting number, and
you specify the increment. In the listing below, I've
specified a space separator, and a width of 6 digits, starting at 100 and
going up in increments of 100.
$ nl -s" " -w6 -v100 -i100
hello.cbl type hello.cbl 100 IDENTIFICATION DIVISION. 200 PROGRAM-ID. HELLO. 300 ENVIRONMENT DIVISION. 400 DATA DIVISION. 500 PROCEDURE DIVISION. 600 PROGRAM-BEGIN. 700 DISPLAY "Hello world." 800 PROGRAM-DONE. 900 STOP RUN.
This is closer, but it still needs work. The number format is controlled by the
-n option. There are three formats. Left-justified with leading zeroes suppressed is represented as
-nln. Right justified with leading zeroes suppressed is
-nrn. (This is the default.) Right-justified with leading zeroes kept is
-nrz. I use
-nrz in the following
$ nl -s" " -w6 -v100 -i100 -nrz
hello.cbl type hello.cbl 000100 IDENTIFICATION DIVISION. 000200 PROGRAM-ID. HELLO. 000300 ENVIRONMENT DIVISION. 000400 DATA DIVISION. 000500 PROCEDURE DIVISION. 000600 PROGRAM-BEGIN. 000700 DISPLAY "Hello world." 000800 PROGRAM-DONE. 000900 STOP RUN.
The default behavior of
nl is to skip blank lines, as shown
above. The treatment of blank lines can be modified with the
-b switch. Some
-b options are
-ba (number all lines),
-bt (number only text lines -- the default behavior), and
-bpstring (number only lines containing the string "string"). This last option is interesting. An artificial example of this is shown in the
following listing. Here, only lines containing the word PROGRAM are numbered.
$ nl -s" " -w6 -v100 -i100 -nrz -bpPROGRAM
hello.cbl type hello.cbl IDENTIFICATION DIVISION. 000100 PROGRAM-ID. HELLO. ENVIRONMENT DIVISION. DATA DIVISION. PROCEDURE DIVISION. 000200 PROGRAM-BEGIN. DISPLAY "Hello world." 000300 PROGRAM-DONE. STOP RUN.
But what we really want is the
-ba option to number all lines. In the
following listing we have the final version of the command, and the result.
$ nl -s" " -w6 -v100 -i100 -nrz -ba
hello.cbl type hello.cbl 000100 IDENTIFICATION DIVISION. 000200 PROGRAM-ID. HELLO. 000300 ENVIRONMENT DIVISION. 000400 DATA DIVISION. 000500 PROCEDURE DIVISION. 000600 000700 PROGRAM-BEGIN. 000800 DISPLAY "Hello world." 000900 001000 PROGRAM-DONE. 001100 STOP RUN.
nl program sounds deceptively simple at first, but it performs
a wide range of numbering tasks. It also includes switches for recognizing the start of new pages, for numbering pages, and to start numbering at the beginning again so that the lines on each page can
start at one.
About the author
Mo Budlong, president of King Computer Services Inc., specializes in Unix and client/server consulting and training and currently publishes the COBOL Just In Time Course, a crash course for the Year 2000 problem, as well as COBOL Dates and the Year 2000, which offers date solutions. Reach Mo at firstname.lastname@example.org.
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Last modified: Monday, July 27, 1998