chapter: awk

Author: pmarinov

index.ptree

@@ -117,6 +117,8 @@ pages/bibliography.poly.pm
   pages/contributed_scripts.poly.pm
   pages/reference_cards.poly.pm
   pages/sed_micro_primer.poly.pm
+  pages/awk_micro_primer.poly.pm
+  pages/parsing_pathnames.poly.pm
 }
 
 }

pages/awk_micro_primer.poly.pm

@@ -0,0 +1,144 @@
+#lang pollen
+
+◊page-init{}
+◊define-meta[page-title]{Awk Micro-Primer}
+◊define-meta[page-description]{Awk Micro-Primer}
+
+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 few of these here - the ones most
+useful in shell scripts. ◊footnote{Its name derives from the initials
+of its authors, Aho, Weinberg, and Kernighan.}
+
+Awk breaks each line of input passed to it into fields. By default, a
+field is a string of consecutive characters delimited by whitespace,
+though there are options for changing this. Awk parses and operates on
+each separate field. This makes it ideal for handling structured text
+files -- especially tables -- data organized into consistent chunks,
+such as rows and columns.
+
+Strong quoting and curly brackets enclose blocks of ◊command{awk} code
+within a shell script.
+
+◊example{
+# $1 is field #1, $2 is field #2, etc.
+
+echo one two | awk '{print $1}'
+# one
+
+echo one two | awk '{print $2}'
+# two
+
+# But what is field #0 ($0)?
+echo one two | awk '{print $0}'
+# one two
+# All the fields!
+
+
+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.
+
+awk '{print $0}' $filename
+# Prints the entire file!
+# Same effect as:   cat $filename . . . or . . . sed '' $filename
+}
+
+We have just seen the ◊command{awk} print command in action. The only
+other feature of ◊command{awk} we need to deal with here is
+variables. Awk handles variables similarly to shell scripts, though a
+bit more flexibly.
+
+◊example{
+{ total += ${column_number} }
+}
+
+This adds the value of ◊ode{column_number} to the running total of
+◊code{total}. Finally, to ◊code{print "total"}, there is an ◊code{END}
+command block, executed after the script has processed all its input.
+
+◊example{
+END { print total }
+}
+
+Corresponding to the END, there is a BEGIN, for a code block to be
+performed before awk starts processing its input.
+
+◊anchored-example[#:anchor "awk_cnt1"]{Counting Letter Occurrences}
+
+The following example illustrates how awk can add text-parsing tools
+to a shell script.
+
+◊example{
+#! /bin/sh
+# letter-count2.sh: Counting letter occurrences in a text file.
+#
+# Script by nyal [[email protected]].
+# Used in ABS Guide with permission.
+# Recommented and reformatted by ABS Guide 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=85
+
+usage()
+{
+    echo "Usage: letter-count.sh file letters" 2>&1
+    # For example:   ./letter-count2.sh filename.txt a b c
+    exit $E_PARAMERR  # Too few 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:
+# 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.
+}