Formatted output
To display messages on standard output we have so far used
echo. This command can only write a single string on a
single line. A command printf (very similar to
printf() in the language C) is provided to format a
message any way you desire. Use of printf involves
giving it a string, known as the format, as first
argument, followed perhaps by subsequent arguments. The format
string is copied to the standard output with certain changes made.
A simple example is
$ printf "Hello"
Hello$
Note that Hello is precisely what has been printed
- no extra spaces and no Newline character after it, so
that the next dollar prompt follows it immediately.
If a \ (backslash) is encountered, it is treated as
an escape character with the following character
examined, and the pair are replaced by a single character. Not all
of these characters will be interpreted sensibly by all terminals,
especially formfeed and vertical tab.
Perhaps the most common escape sequence you will meet is
\n, to terminate a line:
$ printf "Hello\nthere\n"
Hello
there
When a % ('percent') is included, the following
several characters represent a conversion
specification, to instruct how one of the arguments is to be
displayed. There should be the same number of conversion
specifications as arguments following the format string, and they
are paired up with the arguments in order. The next example
illustrates the use of %d to insert a number into the
output:
$ printf "%d is a square number\n"
64
64 is a square number
The string %% is not a specification, and is
replaced by a single % in the output:
$ printf "%s is %d%%\n" "one half"
50
one half is 50%
Between the % and the conversion character may come
a number indicating the field width in which the
argument should be printed, and preceding this number may come a
- (hyphen) indicating that the argument should be
displayed left-justified within the field (it would by default be
right-justified). If the data is numeric, then following the
% immediately with a + would cause the
number always to be displayed with a leading + or
- sign. Note that if you wish printf to
terminate a line, you must do so yourself by including a
\n within the format string. The following examples
illustrate use of printf:
$ printf "Hello %s\n"
$LOGNAME
Hello chris
The string value of LOGNAME is substituted for
%s.
$ printf "The temperature is %+7d degrees\n"
21
The temperature is +21
degrees
The number 21 is substituted for %d, preceded by a
+ sign, and padded out with blanks to fill 7 character
positions.
$ printf "You are %s\nyour home directory
is: %s\n" \
$( logname ) $HOME
You are chris
your home directory is: /home/ugrad/chris
The string that results from executing the command
logname, and the value of the variable
HOME, are substituted for the two %s
specifications. Note the Newlines within the format
string, and the use of a backslash to continue the statement onto
another line when it becomes long.
Worked example
Write a script to read the standard input, and display each word
from the input, right-justified in one column of width 30. A blank
line (or end of file) will terminate the script.
Solution: This is concerned with formatting, so we
need printf. Use a while loop to
continually read in words until a 'null' one is read in (which
happens with a blank line or end of file).
read X # Read first word
while [ "$X" ] # while a "real" word ...
do
printf "%30s\n" $X # print it ...
read X # and read next one
done
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