Add two line before a string in a file using awk

Question

I want to add two line

allow = alaw
allow = g729

before a string

nat = no

in a file sip.conf (or any text based file). If allow = alaw and allow = g729 already exists before nat = no it should not be added. This script will run every hour. And the line should not be added in the two sections immediately after [general] and [providertrunk0] of the file.

Example file content:

[general]
disallow = all
allow = ulaw
nat = no

[providertrunk0]
disallow = all
allow = ulaw
;allow = alaw
nat = no

secret =
nat = no
progressinband = yes

allow = ulaw
allow = alaw
nat = no
progressinband = yes

disallow = all
allow = ulaw
allow = g729
nat = no
progressinband = yes

expected output

[general]
disallow = all
allow = ulaw
nat = no

[providertrunk0]
disallow = all
allow = ulaw
;allow = alaw
nat = no

secret =
allow = alaw
allow = g729
nat = no
progressinband = yes

allow = ulaw
allow = alaw
allow = g729
nat = no
progressinband = yes

disallow = all
allow = ulaw
allow = alaw
allow = g729
nat = no
progressinband = yes

My attempt

cat addCodec.awk

BEGIN {
    RS=""; ORS="\n\n"; FS=OFS="\n"
    skip["[general]"]
    skip["[providertrunk0]"]
    addCodec = "allow = alaw\nallow = g729"
    tgt = "nat = no"
}
!($1 in skip) {
    for (i=1; i<NF; i++) {
        if ( ($i != addCodec) && ($(i+1) == tgt) ) {
            $i = $i OFS addCodec
        }
    }
}
{ print }

awk -f addCodec.awk sip.conf

this script works if both line does not exist , it will add them. If a single line exist then I am stuck , what if condition I need to use.

Answer 1

I'm not specialist in awk (but python) but $i gives single line and you compare it with string which have two lines "allow = alaw\nallow = g729" - and this is problem. You should check every line separatelly.

I created code which check $(i-1) and $(i-2) and compare separatelly with "allow = alaw" (FIRST) and "allow = g729"(SECOND). This way I split it into three situations

FIRST - exist,  SECOND - missed
FIRST - missed, SECOND - missed
FIRST - missed, SECOND - exist

and add different value to $i or $(i-1)

---

BEGIN {
    RS=""; ORS="\n\n"; FS=OFS="\n"
    skip["[general]"]
    skip["[providertrunk0]"]
    addCodec1 = "allow = alaw"
    addCodec2 = "allow = g729"
    tgt = "nat = no"
}
!($1 in skip) {
    for (i=1; i<NF; i++) {
        if ($(i) == tgt) {
            # FIRST - exist, SECOND - missed
            if ( ($(i-1) == addCodec1) ) {
                $i = addCodec2 OFS $i
            }
            # FIRST - missed, SECOND - missed
            else if ( ($(i-1) != addCodec1) && ($(i-1) != addCodec2) ) {
                $i = addCodec1 OFS addCodec2 OFS $i 
            }
            # FIRST - missed, SECOND - exist
            else if ( ($(i-2) != addCodec1) && ($(i-1) == addCodec2)  ) {
                $(i-1) = addCodec1 OFS $(i-1)
            }
        }
    }
}
{ print }

---

BTW: To make it more unviveral (and simpler) I would run it two times with different arguments.

First to put only "allow = g729" before "nat = no", second to put only "allow = alaw" before "allow = g729".

This way I could run it again to put "third line" befor "allow = alaw" and run it again to put "fourth line" befor "third line", etc.

Answer 2

Modifying your existing attempt. The complexity of looking at the various possible scenarios is encapsulated inside a user defined function fx(). Depending on what index the tgt line is found, the various possible scenarios are charted.

$ cat addCodec.txt
BEGIN {
    RS=""; ORS="\n\n"; FS=OFS="\n"
    skip["[general]"]
    skip["[providertrunk0]"]
    e1 = "allow = alaw"
    e2 = "allow = g729"
    a[e2] = a[e1] = 1
    addCodec = e1 OFS e2
    tgt = "nat = no"
}
!($1 in skip) {
    for (i=1; i<=NF; i++) {
        if ($i "" == tgt) {
          fx(i)
          break
        }
    }
}
{ print }
function fx(i,   cond) {
  cond = (i>2 ? (a[$(i-2)]+0) : 0) "" (i>1 ? (a[$(i-1)]+0) : 0)
  if      ( cond "" == "11" ) { $(i-2) = e1; $(i-1) = e2 }
  else if ( cond "" == "01" ) { $(i-1) = addCodec        }
  else if ( cond    ~  /0$/ ) { $(i) = addCodec OFS $(i) }
}

$ awk -f addCodec.txt file