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GoAWK's CSV and TSV file support

CSV and TSV files are often used in data processing today, but unfortunately you can't properly process them using POSIX AWK. You can change the field separator to , or tab (for example awk -F, or awk '-F\t') but that doesn't handle quoted or multi-line fields.

There are other workarounds, such as Gawk's FPAT feature, various CSV extensions for Gawk, or Adam Gordon Bell's csvquote tool. There's also frawk, which is an amazing tool that natively supports CSV, but unfortunately it deviates quite a bit from POSIX-compatible AWK.

Since version v1.17.0, GoAWK has included CSV support, which allows you to read and write CSV and TSV files, including proper handling of quoted and multi-line fields as per RFC 4180. In addition, GoAWK supports a "named field" construct that allows you to access CSV fields by name as well as number, for example @"Address" rather than $5.

In 2023, for the publication of the second edition of The AWK Programming Language, Brian Kernighan updated the original AWK to support proper parsing of CSV files with the new --csv option. Gawk soon followed suit. As of version 1.24.0, GoAWK supports --csv as an equivalent to the GoAWK-specific -i csv option.

Many thanks to the library of the University of Antwerp, who sponsored this feature in May 2022. Thanks also to Eli Rosenthal, whose frawk tool inspired aspects of the design (including the -i and -o command line arguments).

Links to sections:

CSV input configuration

When in CSV input mode, GoAWK ignores the regular field and record separators (FS and RS), instead parsing input into records and fields using the CSV or TSV format. Fields can be accessed using the standard AWK numbered field syntax (for example, $1 or $5), or using the GoAWK-specific named field syntax.

In addition, in CSV input mode the two-argument form of split() uses CSV field splitting and ignores FS. For example, split("x,\"y,z\"", a) would set a[1] = "x" and a[2] = "y,z". The three-argument form of split() operates as usual.

To enable CSV input mode when using the goawk program, use the --csv or -i mode command line argument (mode must be quoted if it has spaces in it). You can also enable CSV input mode by setting the INPUTMODE special variable in the BEGIN block, or by using the Go API. The full syntax of mode is as follows:

csv|tsv [separator=<char>] [comment=<char>] [header]

As of GoAWK 1.24.0, you can use --csv as a shortcut for -i csv. If you just need CSV input mode without additional configuration, --csv is recommended for portability, as original AWK and Gawk now support that option (as of 2023 versions).

The first field in mode is the format: csv for comma-separated values or tsv for tab-separated values. Optionally following the mode are configuration fields, defined as follows:

  • separator=<char>: override the separator character, for example separator=| to use the pipe character. The default is , (comma) for csv format or \t (tab) for tsv format.
  • comment=<char>: consider lines starting with the given character to be comments and skip them, for example comment=# will ignore any lines starting with # (without preceding whitespace). The default is not to support comments.
  • header: treat the first line of each input file as a header row providing the field names, and enable the @"field" syntax as well as the FIELDS array. This option is equivalent to the -H command line argument. If neither header or -H is specified, you can't use named fields.

CSV output configuration

When in CSV output mode, the GoAWK print statement with one or more arguments ignores OFS and ORS and separates its arguments (fields) and records using CSV formatting. No header row is printed; if required, a header row can be printed in the BEGIN block manually. No other functionality is changed, for example, printf doesn't do anything different in CSV output mode.

NOTE: The behaviour of print without arguments remains unchanged. This means you can print the input line ($0) without further quoting by using a bare print statement, but print $0 will print the input line as a single CSV field, which is probably not what you want. See the example below.

To enable CSV output mode when using the goawk program, use the -o mode command line argument (mode must be quoted if it has spaces in it). You can also enable CSV output mode by setting the OUTPUTMODE special variable in the BEGIN block, or by using the Go API. The full syntax of mode is as follows:

csv|tsv [separator=<char>]

The first field in mode is the format: csv for comma-separated values or tsv for tab-separated values. Optionally following the mode are configuration fields, defined as follows:

  • separator=<char>: override the separator character, for example separator=| to use the pipe character. The default is , (comma) for csv format or \t (tab) for tsv format.

Named field syntax

If the header option or -H argument is given, CSV input mode parses the first row of each input file as a header row containing a list of field names.

When the header option is enabled, you can use the GoAWK-specific "named field" operator (@) to access fields by name instead of by number ($). For example, given the header row id,name,email, for each record you can access the email address using @"email", $3, or even $-1 (first field from the right). Further usage examples are shown below.

Every time a header row is processed, the FIELDS special array is updated: it is a mapping of field number to field name, allowing you to loop over the field names dynamically. For example, given the header row id,name,email, GoAWK sets FIELDS using the equivalent of:

FIELDS[1] = "id"
FIELDS[2] = "name"
FIELDS[3] = "email"

Note that named field assignment such as @"id" = 42 is not yet supported, but this feature may be added later.

Go API

When using GoAWK via the Go API, you can still use INPUTMODE, but it may be more convenient to use the interp.Config fields directly: InputMode, CSVInput, OutputMode, and CSVOutput.

Here's a simple snippet showing the use of the InputMode and CSVInput fields to enable # as the comment character:

prog, err := parser.ParseProgram([]byte(src), nil)
if err != nil { ... }

config := &interp.Config{
    InputMode: interp.CSVMode,
    CSVInput:  interp.CSVInputConfig{Comment: '#'},
}
_, err = interp.ExecProgram(prog, config)
if err != nil { ... }

Note that INPUTMODE and OUTPUTMODE set using Vars or in the BEGIN block will override these settings.

See the full reference documentation for the interp.Config struct.

Examples

Below are some examples using the testdata/csv/states.csv file, which is a simple CSV file whose contents are as follows:

"State","Abbreviation"
"Alabama","AL"
"Alaska","AK"
"Arizona","AZ"
"Arkansas","AR"
"California","CA"
...

Example: output a field by name

To output a field by name (in this case the state's abbreviation):

$ goawk -i csv -H '{ print @"Abbreviation" }' testdata/csv/states.csv
AL
AK
AZ
...

You can also use -i 'csv header' to specify "header mode" instead of -i csv -H -- using -H is slightly less typing, but they're equivalent:

$ goawk -i 'csv header' '{ print @"Abbreviation" }' testdata/csv/states.csv
AL
AK
AZ
...

Example: match a field and count

To count the number of states that have "New" in the name, and then print out what they are:

$ goawk -i csv -H '@"State" ~ /New/ { n++ } END { print n }' testdata/csv/states.csv
4
$ goawk -i csv -H '@"State" ~ /New/ { print @"State" }' testdata/csv/states.csv
New Hampshire
New Jersey
New Mexico
New York

Example: rename and reorder fields

To rename and reorder the fields from State, Abbreviation to abbr, name. Note that the print statement in the BEGIN block prints the header row for the output:

$ goawk -i csv -H -o csv 'BEGIN { print "abbr", "name" } { print @"Abbreviation", @"State" }' testdata/csv/states.csv
abbr,name
AL,Alabama
AK,Alaska
...

Example: convert between formats (explicit field list)

To convert the file from CSV to TSV format (note how we're not using -H, so the header row is included):

$ goawk -i csv -o tsv '{ print $1, $2 }' testdata/csv/states.csv
State	Abbreviation
Alabama	AL
Alaska	AK
...

Example: convert between formats (all fields)

If you want to convert between CSV and TSV format but don't know the number of fields, you can use a field assignment like $1=$1 so that GoAWK reformats $0 according to the output format (TSV in this case). This is similar to how in POSIX AWK a field assignment reformats $0 according to the output field separator (OFS). Then print without arguments prints the raw value of $0:

$ goawk -i csv -o tsv '{ $1=$1; print }' testdata/csv/states.csv
State	Abbreviation
Alabama	AL
Alaska	AK
...

NOTE: It's not correct to use print $0 in this case, because that would print $0 as a single TSV field, which you generally don't want:

$ goawk -i csv -o tsv '{ $1=$1; print $0 }' testdata/csv/states.csv  # INCORRECT!
"State	Abbreviation"
"Alabama	AL"
"Alaska	AK"
...

Example: override separator

To test overriding the separator character, we can use GoAWK to add a comment and convert the separator to | (pipe). We'll also add a comment line to test comment handling:

$ goawk -i csv -o 'csv separator=|' 'BEGIN { printf "# comment\n" } { $1=$1; print }' testdata/csv/states.csv
# comment
State|Abbreviation
Alabama|AL
Alaska|AK
...

Example: skip comment lines

We can process the "pipe-separated values" file generated above, skipping comment lines, and printing the first three state names (accessed by field number this time):

$ goawk -i 'csv header comment=# separator=|' 'NR<=3 { print $1 }' testdata/csv/states.psv
Alabama
Alaska
Arizona

Example: use dynamic field names

Similar to the $ operator, you can also use @ with dynamic values. For example, if there are fields named address_1, address_2, up through address_5, you could loop over them as follows:

$ cat testdata/csv/address5.csv
name,address_1,address_2,address_3,address_4,address_5
Bob Smith,123 Way St,Apt 2B,Township,Cityville,United Plates
$ goawk -i csv -H '{ for (i=1; i<=5; i++) print @("address_" i) }' testdata/csv/address5.csv
123 Way St
Apt 2B
Township
Cityville
United Plates

Example: use the FIELDS array

A somewhat contrived example showing use of the FIELDS array to show a numbered list of all fields:

$ cat testdata/csv/fields.csv
id,name,email
1,Bob,[email protected]
$ goawk -i csv -H '{ for (i=1; i in FIELDS; i++) print i, FIELDS[i]; exit }' testdata/csv/fields.csv
1 id
2 name
3 email

Example: create CSV file from array

The following example shows how you might pull fields out of an integer-indexed array to produce a CSV file:

$ goawk -o csv 'BEGIN { print "id", "name"; names[1]="Bob"; names[2]="Jane"; for (i=1; i in names; i++) print i, names[i] }'
id,name
1,Bob
2,Jane

Example: create CSV file by assigning fields

This example shows the same result, but producing the CSV output by assigning individual fields and then using a bare print statement:

$ goawk -o csv 'BEGIN { print "id", "name"; $1=1; $2="Bob"; print; $1=2; $2="Jane"; print }'
id,name
1,Bob
2,Jane

Example: different ways to specify CSV mode

And finally, four equivalent examples showing different ways to specify the input mode, using -i or the INPUTMODE special variable (the same techniques work for -o and OUTPUTMODE):

$ goawk -i csv -H '@"State"=="New York" { print @"Abbreviation" }' testdata/csv/states.csv
NY
$ goawk -icsv -H '@"State"=="New York" { print @"Abbreviation" }' testdata/csv/states.csv
NY
$ goawk 'BEGIN { INPUTMODE="csv header" } @"State"=="New York" { print @"Abbreviation" }' testdata/csv/states.csv
NY
$ goawk -v 'INPUTMODE=csv header' '@"State"=="New York" { print @"Abbreviation" }' testdata/csv/states.csv
NY

Examples based on csvkit

The csvkit suite is a set of tools that allow you to quickly analyze and extract fields from CSV files. Each csvkit tool allows you to do a specific task; GoAWK is more low-level and verbose, but also a more general tool (csvsql being the exception!). GoAWK also runs significantly faster than csvkit (the latter is written in Python).

Below are a few snippets showing how you'd do some of the tasks in the csvkit documentation, but using GoAWK (the input file is testdata/csv/nz-schools.csv):

csvkit example: print column names

$ csvcut -n testdata/csv/nz-schools.csv
  1: School_Id
  2: Org_Name
  3: Decile
  4: Total

# In GoAWK you have to loop through the fields, but you can print the data in
# any format you want (note the "exit" so it stops after the first row):
$ goawk -i csv '{ for (i=1; i<=NF; i++) printf "%3d: %s\n", i, $i; exit }' testdata/csv/nz-schools.csv
  1: School_Id
  2: Org_Name
  3: Decile
  4: Total

# You could also use -H and the FIELDS array to do this:
$ goawk -i csv -H '{ for (i=1; i in FIELDS; i++) printf "%3d: %s\n", i, FIELDS[i]; exit }' testdata/csv/nz-schools.csv
  1: School_Id
  2: Org_Name
  3: Decile
  4: Total

csvkit example: select a subset of columns

$ csvcut -c Org_Name,Total testdata/csv/nz-schools.csv
Org_Name,Total
Waipa Christian School,60
Remarkables Primary School,494
...

# In GoAWK you need to print the field names explicitly in BEGIN:
$ goawk -i csv -H -o csv 'BEGIN { print "Org_Name", "Total" } { print @"Org_Name", @"Total" }' testdata/csv/nz-schools.csv
Org_Name,Total
Waipa Christian School,60
Remarkables Primary School,494
...

# But you can also change the column names and reorder them:
$ goawk -i csv -H -o csv 'BEGIN { print "# Students", "School" } { print @"Total", @"Org_Name" }' testdata/csv/nz-schools.csv
# Students,School
60,Waipa Christian School
494,Remarkables Primary School
...

csvkit example: generate statistics

There's no equivalent of the csvstat tool in GoAWK, but you can calculate statistics yourself. For example, to calculate the total number of students in New Zealand schools, you can do the following (csvstat is giving a warning due to the single-column input):

$ csvcut -c Total testdata/csv/nz-schools.csv | csvstat --sum
/usr/local/lib/python3.9/dist-packages/agate/table/from_csv.py:74: RuntimeWarning: Error sniffing CSV dialect: Could not determine delimiter
802,516

$ goawk -i csv -H '{ sum += @"Total" } END { print sum }' testdata/csv/nz-schools.csv
802516

To calculate the average (mean) decile level for boys' and girls' schools (sorry, boys!):

$ csvgrep -c Org_Name -m Boys testdata/csv/nz-schools.csv | csvcut -c Decile | csvstat --mean
/usr/local/lib/python3.9/dist-packages/agate/table/from_csv.py:74: RuntimeWarning: Error sniffing CSV dialect: Could not determine delimiter
6.45
$ csvgrep -c Org_Name -m Girls testdata/csv/nz-schools.csv | csvcut -c Decile | csvstat --mean
/usr/local/lib/python3.9/dist-packages/agate/table/from_csv.py:74: RuntimeWarning: Error sniffing CSV dialect: Could not determine delimiter
8.889

$ goawk -i csv -H '/Boys/  { d+=@"Decile"; n++ } END { print d/n }' testdata/csv/nz-schools.csv 
6.45
$ goawk -i csv -H '/Girls/ { d+=@"Decile"; n++ } END { print d/n }' testdata/csv/nz-schools.csv 
8.88889

Performance

The performance of GoAWK's CSV input and output mode is quite good, on a par with using the encoding/csv package from Go directly, and much faster than the csv module in Python. CSV input speed is significantly slower than frawk, though CSV output speed is significantly faster than frawk.

Below are the results of some simple read and write benchmarks using goawk and frawk as well as plain Python and Go. The output of the write benchmarks is a 1GB, 3.5 million row CSV file with 20 columns (including quoted columns); the input for the read benchmarks uses that same file. Times are in seconds, showing the best of three runs on a 64-bit Linux laptop with an SSD drive:

Test goawk frawk Python Go
Reading 1GB CSV 3.18 1.01 13.4 3.22
Writing 1GB CSV 5.64 13.0 17.0 3.24

Future work

  • Consider adding a printrow(a) or similar function to make it easier to construct CSV rows from scratch.
    • a would be an array such as: a["name"] = "Bob"; a["age"] = 7
    • keys would be ordered by OFIELDS (eg: OFIELDS[1] = "name"; OFIELDS[2] = "age") or by "smart name" if OFIELDS not set ("smart name" meaning numeric if a keys are numeric, string otherwise)
    • printrow(a) could take an optional second fields array arg to use that instead of the global OFIELDS
  • Consider allowing -H to accept an optional list of field names which could be used as headers in the absence of headers in the file itself (either -H=name,age or -i 'csv header=name,age').
  • Consider adding TrimLeadingSpace CSV input option. See: #109
  • Consider supporting @"id" = 42 named field assignment.

Feedback

Please open an issue if you have bug reports or feature requests for GoAWK's CSV support.