Examples

$ mlr --usage-data-format-examples
  DKVP: delimited key-value pairs (Miller default format)
  +---------------------+
  | apple=1,bat=2,cog=3 |  Record 1: "apple" => "1", "bat" => "2", "cog" => "3"
  | dish=7,egg=8,flint  |  Record 2: "dish" => "7", "egg" => "8", "3" => "flint"
  +---------------------+

  NIDX: implicitly numerically indexed (Unix-toolkit style)
  +---------------------+
  | the quick brown     | Record 1: "1" => "the", "2" => "quick", "3" => "brown"
  | fox jumped          | Record 2: "1" => "fox", "2" => "jumped"
  +---------------------+

  CSV/CSV-lite: comma-separated values with separate header line
  +---------------------+
  | apple,bat,cog       |
  | 1,2,3               | Record 1: "apple => "1", "bat" => "2", "cog" => "3"
  | 4,5,6               | Record 2: "apple" => "4", "bat" => "5", "cog" => "6"
  +---------------------+

  PPRINT: pretty-printed tabular
  +---------------------+
  | apple bat cog       |
  | 1     2   3         | Record 1: "apple => "1", "bat" => "2", "cog" => "3"
  | 4     5   6         | Record 2: "apple" => "4", "bat" => "5", "cog" => "6"
  +---------------------+

  XTAB: pretty-printed transposed tabular
  +---------------------+
  | apple 1             | Record 1: "apple" => "1", "bat" => "2", "cog" => "3"
  | bat   2             |
  | cog   3             |
  |                     |
  | dish 7              | Record 2: "dish" => "7", "egg" => "8"
  | egg  8              |
  +---------------------+

DKVP: Key-value pairs

Miller’s default file format is DKVP, for delimited key-value pairs. Example:

$ mlr cat data/small
a=pan,b=pan,i=1,x=0.3467901443380824,y=0.7268028627434533
a=eks,b=pan,i=2,x=0.7586799647899636,y=0.5221511083334797
a=wye,b=wye,i=3,x=0.20460330576630303,y=0.33831852551664776
a=eks,b=wye,i=4,x=0.38139939387114097,y=0.13418874328430463
a=wye,b=pan,i=5,x=0.5732889198020006,y=0.8636244699032729

Such data are easy to generate, e.g. in Ruby with

puts "host=#{hostname},seconds=#{t2-t1},message=#{msg}"

puts mymap.collect{|k,v| "#{k}=#{v}"}.join(',')

or print statements in various languages, e.g.

echo "type=3,user=$USER,date=$date\n";

logger.log("type=3,user=$USER,date=$date\n");

Fields lacking an IPS will have positional index (starting at 1) used as the key, as in NIDX format. For example, dish=7,egg=8,flint is parsed as "dish" => "7", "egg" => "8", "3" => "flint" and dish,egg,flint is parsed as "1" => "dish", "2" => "egg", "3" => "flint".

As discussed in Record-heterogeneity, Miller handles changes of field names within the same data stream. But using DKVP format this is particularly natural. One of my favorite use-cases for Miller is in application/server logs, where I log all sorts of lines such as

resource=/path/to/file,loadsec=0.45,ok=true
record_count=100, resource=/path/to/file
resource=/some/other/path,loadsec=0.97,ok=false

etc. and I just log them as needed. Then later, I can use grep, mlr --opprint group-like, etc. to analyze my logs.

See Reference regarding how to specify separators other than the default equals-sign and comma.

NIDX: Index-numbered (toolkit style)

With --inidx --ifs ' ' --repifs, Miller splits lines on whitespace and assigns integer field names starting with 1. This recapitulates Unix-toolkit behavior.

Example with index-numbered output:

$ cat data/small
a=pan,b=pan,i=1,x=0.3467901443380824,y=0.7268028627434533
a=eks,b=pan,i=2,x=0.7586799647899636,y=0.5221511083334797
a=wye,b=wye,i=3,x=0.20460330576630303,y=0.33831852551664776
a=eks,b=wye,i=4,x=0.38139939387114097,y=0.13418874328430463
a=wye,b=pan,i=5,x=0.5732889198020006,y=0.8636244699032729

$ mlr --onidx --ofs ' ' cat data/small
pan pan 1 0.3467901443380824 0.7268028627434533
eks pan 2 0.7586799647899636 0.5221511083334797
wye wye 3 0.20460330576630303 0.33831852551664776
eks wye 4 0.38139939387114097 0.13418874328430463
wye pan 5 0.5732889198020006 0.8636244699032729

Example with index-numbered input:

$ cat data/mydata.txt
oh say can you see
by the dawn's
early light

$ mlr --inidx --ifs ' ' --odkvp cat data/mydata.txt
1=oh,2=say,3=can,4=you,5=see
1=by,2=the,3=dawn's
1=early,2=light

Example with index-numbered input and output:

$ cat data/mydata.txt
oh say can you see
by the dawn's
early light

$ mlr --nidx --fs ' ' --repifs cut -f 2,3 data/mydata.txt
say can
the dawn's
light

CSV/TSV/etc.

When mlr is invoked with the --csv or --csvlite option, key names are found on the first record and values are taken from subsequent records. This includes the case of CSV-formatted files. See Record-heterogeneity for how Miller handles changes of field names within a single data stream.

Miller has record separator RS and field separator FS, just as awk does. For TSV, use --fs tab; to convert TSV to CSV, use --ifs tab --ofs comma, etc. (See also Reference.)

Miller’s --csv flag supports RFC-4180 CSV ( https://tools.ietf.org/html/rfc4180). This includes CRLF line-terminators by default, regardless of platform. You can use mlr --csv --rs lf for native Un*x (LF-terminated) CSV files.

The RFC says, somewhat briefly, that “there may be a header line”. Miller’s --implicit-csv-header option allows you to read CSV data which lacks a header line, applying column labels 1, 2, 3, etc. for you. You may also use Miller’s label to replace those numerical column names with labels of your choosing.

PPRINT: Pretty-printed tabular

Miller’s pretty-print format is like CSV, but column-aligned. For example, compare

$ mlr --ocsv cat data/small
a,b,i,x,y
pan,pan,1,0.3467901443380824,0.7268028627434533
eks,pan,2,0.7586799647899636,0.5221511083334797
wye,wye,3,0.20460330576630303,0.33831852551664776
eks,wye,4,0.38139939387114097,0.13418874328430463
wye,pan,5,0.5732889198020006,0.8636244699032729

$ mlr --opprint cat data/small
a   b   i x                   y
pan pan 1 0.3467901443380824  0.7268028627434533
eks pan 2 0.7586799647899636  0.5221511083334797
wye wye 3 0.20460330576630303 0.33831852551664776
eks wye 4 0.38139939387114097 0.13418874328430463
wye pan 5 0.5732889198020006  0.8636244699032729

Note that while Miller is a line-at-a-time processor and retains input lines in memory only where necessary (e.g. for sort), pretty-print output requires it to accumulate all input lines (so that it can compute maximum column widths) before producing any output. This has two consequences: (a) pretty-print output won’t work on tail -f contexts, where Miller will be waiting for an end-of-file marker which never arrives; (b) pretty-print output for large files is constrained by available machine memory.

See Record-heterogeneity for how Miller handles changes of field names within a single data stream.

XTAB: Vertical tabular

This is perhaps most useful for looking a very wide and/or multi-column data which causes line-wraps on the screen (but see also https://github.com/twosigma/ngrid for an entirely different, very powerful option). Namely:

$ grep -v '^#' /etc/passwd | head -n 6 | mlr --nidx --fs : --opprint cat
1          2 3  4  5                          6               7
nobody     * -2 -2 Unprivileged User          /var/empty      /usr/bin/false
root       * 0  0  System Administrator       /var/root       /bin/sh
daemon     * 1  1  System Services            /var/root       /usr/bin/false
_uucp      * 4  4  Unix to Unix Copy Protocol /var/spool/uucp /usr/sbin/uucico
_taskgated * 13 13 Task Gate Daemon           /var/empty      /usr/bin/false
_networkd  * 24 24 Network Services           /var/networkd   /usr/bin/false

$ grep -v '^#' /etc/passwd | head -n 2 | mlr --nidx --fs : --oxtab cat
1 nobody
2 *
3 -2
4 -2
5 Unprivileged User
6 /var/empty
7 /usr/bin/false

1 root
2 *
3 0
4 0
5 System Administrator
6 /var/root
7 /bin/sh