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

The short answer is "I wouldn't use gawk to parse CSV if the CSV contains awkward data", where 'awkward' means things like commas in the CSV field data.

The next question is "What other processing are you going to be doing", since that will influence what alternatives you use.

I'd probably use Perl and the Text::CSV or Text::CSV_XS modules to read and process the data. Remember, Perl was originally written in part as an awk and sed killer - hence the a2p and s2p programs still distributed with Perl which convert awk and sed scripts (respectively) into Perl.

Answer 2

If permissible, I would use the Python csv module, paying special attention to the dialect used and formatting parameters required, to parse the CSV file you have.

Answer 3

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