C.2. Awk
Awk is a full-featured text processing
language with a syntax reminiscent of C. While
it possesses an extensive set of operators and capabilities,
we will cover only a couple of these here - the ones most useful
for shell scripting.
Awk breaks each line of input passed to it into
fields. By default, a field is
a string of consecutive characters separated by whitespace, though there are
options for changing the delimiter. Awk parses and operates on
each separate field. This makes awk ideal for handling structured
text files -- especially tables -- data organized into consistent
chunks, such as rows and columns.
Strong quoting (single quotes) and curly brackets enclose
segments of awk code within a shell script.
echo one two | awk '{print $1}'
# one
echo one two | awk '{print $2}'
# two
awk '{print $3}' $filename
# Prints field #3 of file $filename to stdout.
awk '{print $1 $5 $6}' $filename
# Prints fields #1, #5, and #6 of file $filename. |
We have just seen the awk print command
in action. The only other feature of awk we need to deal with
here is variables. Awk handles variables similarly to shell
scripts, though a bit more flexibly.
{ total += ${column_number} } |
This adds the value of
column_number to
the running total of
"total". Finally, to print
"total", there is an
END command
block, executed after the script has processed all its input.
Corresponding to the END, there is a
BEGIN, for a code block to be performed before awk
starts processing its input.
The following example illustrates how awk can
add text-parsing tools to a shell script.
Example C-1. Counting Letter Occurrences
#! /bin/sh
# letter-count2.sh: Counting letter occurrences in a text file.
#
# Script by nyal [[email protected]].
# Used with permission.
# Recommented by document author.
# Version 1.1: Modified to work with gawk 3.1.3.
# (Will still work with earlier versions.)
INIT_TAB_AWK=""
# Parameter to initialize awk script.
count_case=0
FILE_PARSE=$1
E_PARAMERR=65
usage()
{
echo "Usage: letter-count.sh file letters" 2>&1
# For example: ./letter-count2.sh filename.txt a b c
exit $E_PARAMERR # Not enough arguments passed to script.
}
if [ ! -f "$1" ] ; then
echo "$1: No such file." 2>&1
usage # Print usage message and exit.
fi
if [ -z "$2" ] ; then
echo "$2: No letters specified." 2>&1
usage
fi
shift # Letters specified.
for letter in `echo $@` # For each one . . .
do
INIT_TAB_AWK="$INIT_TAB_AWK tab_search[${count_case}] = \"$letter\"; final_tab[${count_case}] = 0; "
# Pass as parameter to awk script below.
count_case=`expr $count_case + 1`
done
# DEBUG:
# echo $INIT_TAB_AWK;
cat $FILE_PARSE |
# Pipe the target file to the following awk script.
# ----------------------------------------------------------------------------------
# Earlier version of script used:
# awk -v tab_search=0 -v final_tab=0 -v tab=0 -v nb_letter=0 -v chara=0 -v chara2=0 \
awk \
"BEGIN { $INIT_TAB_AWK } \
{ split(\$0, tab, \"\"); \
for (chara in tab) \
{ for (chara2 in tab_search) \
{ if (tab_search[chara2] == tab[chara]) { final_tab[chara2]++ } } } } \
END { for (chara in final_tab) \
{ print tab_search[chara] \" => \" final_tab[chara] } }"
# ----------------------------------------------------------------------------------
# Nothing all that complicated, just . . .
#+ for-loops, if-tests, and a couple of specialized functions.
exit $?
# Compare this script to letter-count.sh. |
For simpler examples of awk within shell scripts, see:
Example 11-12
Example 16-8
Example 12-29
Example 33-5
Example 9-23
Example 11-18
Example 27-2
Example 27-3
Example 10-3
Example 12-55
Example 9-28
Example 12-4
Example 9-13
Example 33-16
Example 10-8
Example 33-4
That's all the awk we'll cover here, folks, but there's lots
more to learn. See the appropriate references in the Bibliography.
