Parsing a CSV file using gawk

Question

How do you parse a CSV file using gawk? Simply setting FS=\",\" is not enough, as a quoted field with a comma inside will be treated as multiple fields.

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Answer 1

csv2delim.awk

# csv2delim.awk converts comma delimited files with optional quotes to delim separated file
#     delim can be any character, defaults to tab
# assumes no repl characters in text, any delim in line converts to repl
#     repl can be any character, defaults to ~
# changes two consecutive quotes within quotes to '

# usage: gawk -f csv2delim.awk [-v delim=d] [-v repl=`"] input-file > output-file
#       -v delim    delimiter, defaults to tab
#       -v repl     replacement char, defaults to ~

# e.g. gawk -v delim=; -v repl=` -f csv2delim.awk test.csv > test.txt

# abe 2-28-7
# abe 8-8-8 1.0 fixed empty fields, added replacement option
# abe 8-27-8 1.1 used split
# abe 8-27-8 1.2 inline rpl and "" = '
# abe 8-27-8 1.3 revert to 1.0 as it is much faster, split most of the time
# abe 8-29-8 1.4 better message if delim present

BEGIN {
    if (delim == "") delim = "\t"
    if (repl == "") repl = "~"
    print "csv2delim.awk v.m 1.4 run at " strftime() > "/dev/stderr" ###########################################
}

{
    #if ($0 ~ repl) {
    #   print "Replacement character " repl " is on line " FNR ":" lineIn ";" > "/dev/stderr"
    #}
    if ($0 ~ delim) {
        print "Temp delimiter character " delim " is on line " FNR ":" lineIn ";" > "/dev/stderr"
        print "    replaced by " repl > "/dev/stderr"
    }
    gsub(delim, repl)

    $0 = gensub(/([^,])\"\"/, "\\1'", "g")
#   $0 = gensub(/\"\"([^,])/, "'\\1", "g")  # not needed above covers all cases

    out = ""
    #for (i = 1;  i <= length($0);  i++)
    n = length($0)
    for (i = 1;  i <= n;  i++)
        if ((ch = substr($0, i, 1)) == "\"")
            inString = (inString) ? 0 : 1 # toggle inString
        else
            out = out ((ch == "," && ! inString) ? delim : ch)
    print out
}

END {
    print NR " records processed from " FILENAME " at " strftime() > "/dev/stderr"
}

test.csv

"first","second","third"
"fir,st","second","third"
"first","sec""ond","third"
" first ",sec   ond,"third"
"first" , "second","th  ird"
"first","sec;ond","third"
"first","second","th;ird"
1,2,3
,2,3
1,2,
,2,
1,,2
1,"2",3
"1",2,"3"
"1",,"3"
1,"",3
"","",""
"","""aiyn","oh"""
"""","""",""""
11,2~2,3

test.bat

rem test csv2delim
rem default is: -v delim={tab} -v repl=~
gawk                      -f csv2delim.awk test.csv > test.txt
gawk -v delim=;           -f csv2delim.awk test.csv > testd.txt
gawk -v delim=; -v repl=` -f csv2delim.awk test.csv > testdr.txt
gawk            -v repl=` -f csv2delim.awk test.csv > testr.txt