Day 3: AWK builtins & defaults, basic IO, printf() / sprintf()

AWK has several built-in variables; most can be reassigned

# AWK built-in Variables and (default value):
# 
# FILENAME = name of current data file if any
# 
# NF = number of fields in current record
# NR = number of records, total 
# FNR = number of records, current file
#  => NF, NR, FNR set with each new record read
#  => NF can be reassigned, i.e.
#      $ echo 'a b c' |awk '{NF-- ; print $0}'
#      a b
# 
# FS = field separator (" " ; can be regex)
# OFS = output field separator (" ")
# 
# RS = record separator ("\n") *
# ORS = output record separator ("\n")
#   * RegEx sometimes supported; POSIX limits to single char
# 
# OFMT = floating-point format string ("%.6g")
# CONVFMT = floating-point -> str  format string ("%.6g")
# 
# RSTART = start of pattern match in match()
# RLENGTH = length of pattern match in match()
# SUBSEP = "multi-dim" array indices separator ("\034")
# ARGC = # of cmd line args (includes calling cmd)
# 

AWK has two built-in arrays: ENVIRON & ARGV

ENVIRON = array of inherited shell environment variables

#  ex. print PWD env. var:
#    $ awk 'BEGIN{if(PWD in ENVIRON)print "PWD =",ENVIRON["PWD"]}'
#    PWD = /home/sdfer/Projects/AWK
#

ENVIRON["var"] reassignments won't affect inherited environment

ARGV = array of cmd line args ; ARGV[0] = "awk" ; ARGC = total

can indirectly access vars assigned as args in BEGIN{} via ARGV
 => see example in Day1.log


AWK has several built-in commands => see POSIX standard reference

AWK command sampling..

index(s, t)  => returns position of "t" in "s" OR 0 if not found

match(s, e)  => returns position of "e" in "s" OR 0 if not found;
		"e" can be string OR regex; sets RSTART & RLENGTH
		variables; often used with substr()

substr(s, m[, n]) => returns portion of "s" from 'm' to end, or 'n'

# ex. index(), match(), substr() sampling..
#    $ echo 'a_b_c' |awk '!index($0,"Z"){print " => no Z"}'
#    no Z
#    $ echo 'a_b_c' |awk '{print substr($0,index($0,"b"))}'
#    b_c
#    $ echo 'a_b_c' |awk '{print substr($0,index($0,"b")-1,3)}'
#    _b_
#    $ echo 'a_b_c' |awk '{match($0,"b");print substr($0,RSTART)}'
#    b_c
#    $ echo 'a_b_c' |awk '{match($0,"b");print substr($0,RSTART,1)}'
#    b
#    $ echo 'a_b_c' |awk '{match($0,".b.");print substr($0,RSTART,RLENGTH)}'
#    _b_
#    $ echo 'a_b_c' |awk '{match($0,".b.");print substr($0,++RSTART,RLENGTH-2)}'
#    b
#

AWK has two distructive commands: sub() and gsub()

sub(e, r[, v])  => replace "e" with "r" in $0 or 'v' ; just once

gsub(e, r[, v]) => replace "e" with "r" in $0 or 'v' ; multiple times

for both, "e" can be string or regex ; assumes $0 if 'v' omitted

# ex. sub() and gsub() sample..
#    $ echo 'a - b - c' |awk '{sub("-", "+")};//'
#    a + b - c
#    $ echo 'a - b - c' |awk '{sub("-", "+", $2)};//'
#    a + b - c
#    $ echo 'a - b - c' |awk '{sub("-", "+", $4)};//'
#    a - b + c
#    $ echo 'a - b - c' |awk '{gsub("-", "+")};//'
#    a + b + c
#    $ echo 'a - b - c' |awk '{gsub("[a-z]", "&&&")};//'
#    aaa - bbb - ccc
#    $ echo 'a - b - c' |awk '{gsub("[a-z]", $2)};//'
#    - - - - -
#

... onto IO ...

AWK special filenames: "/dev/stdin", "/dev/stdout", "/dev/stderr"
 => for POSIX systems; check manpage for proper referencing

terminal IO: "-" == "/dev/stdin" ; "/dev/tty" == "/dev/stdout"

#  eg. woot all the way down..
#
#    $ echo 'woot' |awk '//'
#     woot
#     $ echo 'woot' |awk '//' -
#     woot
#     $ echo 'woot' |awk '//' /dev/stdin
#     woot
#     $ echo 'woot' |awk '{print}'
#     woot
#     $ echo 'woot' |awk '{print > "/dev/tty"}'
#     woot
#     $ echo 'woot' |awk '{print > "/dev/stdout"}'
#     woot
#     


AWK can use '<', '>', '>>', and '|' for external files & cmds

#  ex.  random data sorted numerically assending:
#
#    $ shuffle -n7 |awk '{print |"sort"}END{close("sort")}'
#    0
#    1
#    2
#    3
#    4
#    5
#    6
#

note: the first use of 'print >"file"' zeros, subsequent use appends
 => safer to use "shell rules" and stick to '>>' for appending

note: while AWK supports "/dev/stderr" it mostly lacks error handling
 => ie. no way to determine if a file is writable beforehand

for finer-grained control use test(1) via system() instead:

#  ex. test if file-ro is writable:
#    $ awk '!system("test -w" FILENAME){print "not writable"}' test-ro
#    not writable
#
#  note: system() only returns it's exit status
#

... onto printf() / sprintf() ...

AWK's printf() is modelled on printf(3) from C language

basic form:  printf ("format_str", arg, arg, ...)  ; '()' are optional

AWK sprintf() similar but used for string creation ; '()' are required

both printf() and sprintf() support dynamic field sizing:

# ex. print f.p. value w/ 7 sig. fig. & percision of 2, 0 padded:
#    $ awk 'BEGIN{printf "%0*.*f\n", 7, 2, 123.456789}'
#    0123.46
#
#  note: zero padding only works w/ numbers
#

printf() quoted format strings can be assigned to variables and used
 => $ awk 'BEGIN{Fmt="%0*.*f\n"; printf Fmt, 7, 2, 123.456789}'

sprintf() often useful for string concatenation

#  ex. string concatenation 2 different ways..
#    $ awk 'BEGIN{S = "a" ":" "b" ; print S}'
#    a:b
#    $ awk 'BEGIN{S = sprintf("%s:%s", "a", "b") ; print S}'
#    a:b
#