How do I append variable name while splitting fastq file?

I have a fastq file below and I want to split the file by lane=$2. My code does the job of splitting it, but I also want the output files to have $SM variable appended to them. Can someone please let me know what I am missing in my command?

SM="sample1"
awk 'BEGIN {FS = ":"} {lane=$2 ; print > "${SM}."lane".fastq" ; for (i = 1; i <= 3; i++) {getline ; print > "${SM}."lane".fastq"}}' < File.fastq

File.fastq

@HS2000-1015_160:7:1108:13370:100570/2
CTTGACTGCCAGAGACGCTCCTTTGCAATGCCTTCCGGTAACCAAATTTTTGGGCACAACACACAGCTGGCCTTCATTTCTTCAGGGGCTGGTAAACAGA
+
@@@ADFFFHHHFD=EF@:GHIIFHH<ECHGF@DDBB:6@D60?F=888)8='--(=5@EAE5?'(..((.;?@>>A>3;@####################
@HS2000-1015_160:5:2306:10070:71746/2
GAACCTCAAGGACTATTGGGAGAGCGGCGAGTGGGCCATCATCAAAGCCCCAGGCTACAAACACGACATCAAGTACAACTGCTGCGAGGAGATCTACCCC
+
@CCFFFFDHGHFHIJJJJJJGGGIIJJIGHI@FHGIIGHHEFGHHFFFFFBCDDDDDCDDDDDDDD;@BDCCDACDD@>ACCDDDDBDB<BA?C@CC@BD
@HS2000-1015_160:6:2116:4077:79041/2
GGTCCCCGCCTACGCCCACTGGGTTGGTGCACCTGGTGGTGGTGGCCGCCAAGAAGCTGGTGAACCGCCTCCAAGTGGCTCCCAAGACGCAGCTGGATGA
+
CCCFFFFFHHFHHJHJJJJJJJJGGHJHIGAGIIJIFHJ;@F;CHHFHFDDDDDCDDCDD9CCCDDBDDBBDDCDACDD8@BD3>?BCDBDDDACCDC@>
@HS2000-1015_160:5:2113:11446:94436/2
CGTCAGGGCCAACCCCGCCCCACCCTGACCCTACCTGGCACCCCTCACCTGTGGCCTGCCAGCACAGCCTCGCCCCTGCTGGCCAATGTGTCCCCCGTCA
+
?@@DA@DDFHH?DHI)<@@FHDBGGCHCBDH;DFA<)6.=7D;@CBCHD)).7@=>;?==AABC95<(5(5309@D########################
@HS2000-1015_160:6:2209:18284:44195/2
TAAAATGTCACAAAGCTGGAAACTCTTCCCTATCACAAACCAAAACTTAAAAGGACGTTACCTGGCTGGGTCTAAACTCCACATAACTCGCTTGCAGTTG
+
CCCFFFFEHHHGHJIIIJJIJJHIIJEHJJHIJJJIIJJIJIJJIJIIHJJIJGGHGHGIIHHIIIIHFH@DFFFDEEEECDDDCDDDDBDDBBDCDACC
@HS2000-1015_160:7:1215:18781:100685/2
ATAAAACAGTAAACAAAATAAAGTCAGTTTTTTTTTTTTTTTTTAAAGAACAAAATGAAACTTGAGGGAAAACTTCATGGAGTTACAGTTTATCCTGATA
+
CCCFFFFFHFHHFJJJJIIGIGI<CFHHIIJJJJJIJJHFDDD=ACC(38+9CB?:(>C(+:@>(4?05<?C?###########################
@HS2000-1015_160:6:1215:6292:43622/2
GGGTCCTGAGACCTGAGGGACCATTGGCCCTCTTCTGGCTTGCTTATCCTTTGTACCTGATGGCCAATGAATGTCAGAGATGGTCCTGTCTCCATCCAGT
+
BCCDFFFFHGHHHJJJIIJJJJJIJJJGIJIJJIHIIJJIEFHEIJJJJIGIGIIIIIJHFHIJJJJIHGHEC?BCEFFFEECCCEACCCCDDDDDDCCC
@HS2000-1015_160:7:2311:1291:4696/2
GATCTGGTGCTCGTATTCCATCCACCTCCCAAGCTATACATAATAACGGCCAAAGGACCTGGATGAAAGTGTCTGAAGCAGTTGTGTGTGTCTCACCTTC
+
?=?ABDDBCFDFHGGHBFCHHGD@GFDGCBDFGFFECCHHD@DDFHJEIIHGG3CE9C(7@E(.7=?;;@C?@ECA>@C3A(;A-5595<9:AC3@AC:A
@HS2000-1015_160:7:1205:18979:53766/2
TCTTGTTTTGACCAATAGTAAAGCACATTTCTCTAATTTGGATTTCTACAATATCCATATCTTGGTTTATGAAAGGTAGGGAAGAGACTTCAGGTACTGC
+
CCCFFDFFHHHHHIJIJJJIHIJHJJIJJJJIJIGIIIJJJJJHJIJJIJDHIJIIIIIJJJJIJGIJJJIIIGEEGCD@AHHFFEDFFCDDDDCCDD@C
@HS2000-1015_160:7:1205:5641:24287/2
ATAAGAAGGGAAGAATGATTAGGTGTCAAATGTTCTTTTTATTTTCTTTCAGTTCAATGCAAAAACTTTCCAGTGATTATGTAAATGCAGAATCATGTGG
+
CCCFFFFFHHHGHJIJJFJJGIGEHEHIJJJJGJGJJIJJJJJJJJJJJIJIIIJJJIJJIEHGIHGJJJJIGGGHIIIIEEEHCHHC>DFBEEA@CCCC
@HS2000-1015_160:7:1310:19879:73973/2
TTCTTGAGTTCTGATACCTGTTTCCACAATCGTTTCTGTTTCTGTTGTCTCCAGCCCATCCATGCTGTCCTCATCTTCCACTGCAGTTTTCACCCTACTT
+
@<@FFFDFHHH>FGGIJAEFHABHHIAGHAE=F@EF?FB@F:F<GGBGEHGGG9F=BGAGIIIHH;=.=CHG@CEHE3)7?=>)7@C>)(6(.6;A?ACC
@HS2000-1015_160:7:1215:4243:29984/2
ATCTACACCCAAAACAGAACTTTCACAAAAAAACTGTTGATACGAAGCTCATGAAAATCATGATGAATACTCCAACAATTAATGAATAAAACTATACAAT
+
;@@A;D;ADDFHFIIF3EG@A>ACEHE>EH=:DH@<9DB@F?B7C87'@)=)7@>@7==)7...).;?@C)6;((;(5;(>A:(:3;@3>:@>:@(4@::
@HS2000-1015_160:7:1314:6987:62989/2
ATAGCTGTCTGTTCAGAGTCTGATGTTTTCAGTAACACTCTTGATACATTAAGTGAGATAGAATGGAATCCAGCAACAAAGCTACTAAATCAGGTAACTT
+
C@CFFFFDHHHHHJIJJJBHHIIIIHJIJHGJIJJIEHGHJJIJJJJJJJJIGBGHHIJGHGIIHJJIJIIJIGIGHIGGGCHHHHBEFCCEFE>CCEEE
@HS2000-1015_160:6:1208:20370:97766/2
TTTACTTTTTCCCAAACAATAATGATGATAATGTGGCCATACTGGTGCATGAGGGCTCTTATTAAGGATAGGGGCCATGTCAGGCTCTATTGACTCCTAT
+
CCCFFFFFDHDFHJJJIJJJIIJGHJJJIIIIGHIJJIJJJIJIHIJJIIHGHIFHIFHJGIJJIJJJJJJJJHHHFFFFFEEEEEDDCDEDDDDDDCDD
@HS2000-1015_160:6:1108:20693:2521/2
CCCATTTTCTGATGAGGAAACAGGATCAGGGACATTGAGACCTACCAAAGTTACATAATACCAGTAGTAGAAATGGGACTTCAACACAGGCCTCTTGACT
+
7@@DDDDDHHHBDIGIB@F?A+AF@3+2AFE@1:BFE??HH6?BG9BD99??F49BC=88=:;F8=77/@EH=EHF9)=A>C>7?;(6@???C?>@####
@HS2000-1015_160:6:1206:11472:64908/2
AGTTTGTTGGACATTTGAGACCCCAGGAAATCCCCTTTCTCGTAACGTTCTCCGCTTGGATCTGATCTCAACAGGGTGTCGTAGTCATTCTTCAGCACAA
+
B@BDFFFFHHHHHIJGIIJIJJIJJJJGEGHHIJJJJJJIJIFFHIIHCHHIJJJGIIJH:CHHFFFFFFFEEEDD=@BDDDAB@DCDDDDDDD>CCB<?
@HS2000-1015_160:7:1114:4995:49287/2
CCTCCGCTCAGCACTGGCATTGGCATCGGTTTCTATGGCAACAGTGAGACCAGTGATGGGGTGTCCCAGCTCAGCTCTGCGCTGCTGCACGCCAACCACA
+
BCCFDFFFHHHHHJJJJJGHEIIJHIGIIFGHGIIIGHEHIIJJDHIJJJJJJEGIGGIDE:?BCEEAE@CCDCDDCDDDDDDDBCCDDD85?9BB@BDD
@HS2000-1015_160:7:1206:16723:26612/2
TTAGATATGCTGTATGTGAAGAAGAGGAGGTTAAAGAACACTGTTTTATGTAAATGTCTCATTCCTTATCCTACAGAAATTGCATTTTTAATTAAATCTT
+
BC@FFFFFHHHHHICIGGHEIGJJIJIEGHGHIJJGGIIIIJIFGIJJIIJIIIJJIIJJJJJIHHGJJGIIIIGIIIHIIFHGHFADFFFDFDE(;@CE
@HS2000-1015_160:5:2101:1745:52266/2
CCCCAGAATTCTCTTGTTTTTTCCTTGGTGATCCAGGAAAACGAAGCCCCCTCCTGTATTGACAGCTGGGAATTGTGGAGTCCACCGTCCTCCACCTGAG
+
C@CFFFFFHHHHHJIJJIJJJJJIIICHCEGIIIEHGIIHIJIGGGIJCHGIHHHGEFHHHGHEEFFDEDAC?CDDCDCD>95>:,,99@DCC?<AB9AC

Result file names I am getting:

${SM}.5.fastq
${SM}.6.fastq
${SM}.7.fastq

Result file names I want:

sample1.5.fastq 
sample1.6.fastq
sample1.7.fastq

Solution

EDIT: As per OP's comment adding solution(improved one) including the output file name changing.

SM="sample1"
awk -v sm="$SM" '
BEGIN{FS = ":"} 
/^@HS/{
  split($1,arr,"_")
  sub(/^@[a-zA-Z]+/,"",arr[1])
  lane=$2
  close(outputFile)  
  outputFile=sm"."arr[1]"."lane".fastq"
}
{
  print >> (outputFile)
}' File.fastq

Fixing OP's attempt: Could you please try following, you could actually use -v awk_var_name="$shell_var" for which I shared link in comments section too, I have also fixed few things too in your code.

SM="sample1"
awk -v sm="$SM" '
BEGIN{FS = ":"} 
{
  close(outputFile)
  lane=$2
  outputFile=sm count "."lane".fastq"
  print > (outputFile)
  for (i = 1; i <= 3; i++){getline ; print  > (outputFile)}
}' File.fastq

Fixes in OP's attempts:

Created outputFile variable which has the output file name for clarity purposes.
Used close command to close the output file, so that we don't get too many file opened error
As per experts getline is not much recommended so changed that approach to checking the line number check by doing FNR%4==0

Ideal way could be:

SM="sample1"
awk -v sm="$SM" '
BEGIN{FS = ":"} 
/^@HS/{
  lane=$2
  close(outputFile)  
  outputFile=sm count "."lane".fastq"
}
{
  print >> (outputFile)
}' File.fastq