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Record, field, and pair separators

Miller has record separators, field separators, and pair separators. For example, given the following DKVP records:

cat data/a.dkvp
  • the record separator is newline -- it separates records from one another;
  • the field separator is , -- it separates fields (key-value pairs) from one another;
  • and the pair separator is = -- it separates the key from the value within each key-value pair.

These are the default values, which you can override with flags such as --ips and --ops (below).

Not all file formats have all three of these: for example, CSV does not have a pair separator, since keys are on the header line and values are on each data line.

Also, separators are not programmable for all file formats. For example, in JSON objects, the pair separator is : and the field-separator is , -- we write {"a":1,"b":2,"c":3} -- but these aren't modifiable. If you do mlr --json --ips : --ips '=' cat myfile.json then you don't get {"a"=1,"b"=2,"c"=3}. This is because the pair-separator : is part of the JSON specification.

Input and output separators

Miller lets you use the same separators for input and output (e.g. CSV input, CSV output), or, to change them between input and output (e.g. CSV input, JSON output), if you wish to transform your data in that way.

Miller uses the names IRS and ORS for the input and output record separators, IFS and OFS for the input and output field separators, and IPS and OPS for input and output pair separators.

For example:

cat data/a.dkvp
mlr --ifs , --ofs ';' --ips = --ops : cut -o -f c,a,b data/a.dkvp
mlr --csv head -n 2 example.csv
mlr --csv --ofs pipe head -n 2 example.csv

If your data has non-default separators and you don't want to change those between input and output, you can use --rs, --fs, and --ps. Setting --fs : is the same as setting --ifs : --ofs :, but with fewer keystrokes.

cat data/modsep.dkvp
mlr --fs ';' --ps : cut -o -f c,a,b data/modsep.dkvp

Multi-character separators

All separators can be multi-character, except for file formats which don't allow parameterization (see below). And for CSV (CSV-lite doesn't have these restrictions), IRS must be \n and IFS must be a single character.

mlr --ifs ';' --ips : --ofs ';;;' --ops := cut -o -f c,a,b data/modsep.dkvp

If your data has field separators which are one or more consecutive spaces, you can use --ifs space --repifs. More generally, the --repifs flag means that multiple successive occurrences of the field separator count as one. For example, in CSV data we often signify nulls by empty strings, e.g. 2,9,,,,,6,5,4. On the other hand, if the field separator is a space, it might be more natural to parse 2 4 5 the same as 2 4 5: --repifs --ifs ' ' lets this happen. In fact, the --ipprint option is internally implemented in terms of --repifs.

For example:

cat data/extra-spaces.txt
oh    say   can you
see   by    the dawn's
early light what so
mlr --ifs ' ' --repifs --inidx --oxtab cat  data/extra-spaces.txt
1 oh
2 say
3 can
4 you

1 see
2 by
3 the
4 dawn's

1 early
2 light
3 what
4 so

Regular-expression separators

IFS and IPS can be regular expressions: use --ifs-regex or --ips-regex in place of --ifs or --ips, respectively.

You can also use either --ifs space --repifs or --ifs-regex '()+'. (But that gets a little tedious, so there are aliases listed below.) Note however that --ifs space --repifs is about 3x faster than --ifs-regex '( )+' -- regular expressions are powerful, but slower.


Many things we'd like to write as separators need to be escaped from the shell -- e.g. --ifs ';' or --ofs '|', and so on. You can use the following if you like:

mlr help list-separator-aliases
ascii_esc  = "\x1b"
ascii_etx  = "\x04"
ascii_fs   = "\x1c"
ascii_gs   = "\x1d"
ascii_null = "\x01"
ascii_rs   = "\x1e"
ascii_soh  = "\x02"
ascii_stx  = "\x03"
ascii_us   = "\x1f"
asv_fs     = "\x1f"
asv_rs     = "\x1e"
colon      = ":"
comma      = ","
cr         = "\r"
crcr       = "\r\r"
crlf       = "\r\n"
crlfcrlf   = "\r\n\r\n"
equals     = "="
lf         = "\n"
lflf       = "\n\n"
newline    = "\n"
pipe       = "|"
semicolon  = ";"
slash      = "/"
space      = " "
tab        = "\t"
usv_fs     = "\xe2\x90\x9f"
usv_rs     = "\xe2\x90\x9e"

And for --ifs-regex and --ips-regex:

mlr help list-separator-regex-aliases
spaces     = "( )+"
tabs       = "(\t)+"
whitespace = "([ \t])+"

Note that spaces, tabs, and whitespace already are regexes so you shouldn't use --repifs with them. (In fact, the --repifs flag is ignored when --ifs-regex is provided.)

Command-line flags

Given the above, we now have seen the following flags:

--rs --irs --ors
--fs --ifs --ofs --repifs --ifs-regex
--ps --ips --ops --ips-regex

See also the separator-flags section.

DSL built-in variables

Miller exposes for you read-only built-in variables with names IRS, ORS, IFS, OFS, IPS, and OPS. Unlike in AWK, you can't set these in begin-blocks -- their values indicate what you specified at the command line -- so their use is limited.

mlr --ifs , --ofs ';' --ips = --ops : --from data/a.dkvp put '$d = ">>>" . IFS . "|||" . OFS . "<<<"'

Which separators apply to which file formats


  • CSV IRS and ORS must be newline, and CSV IFS must be a single character. (CSV-lite does not have these restrictions.)
  • TSV IRS and ORS must be newline, and TSV IFS must be a tab. (TSV-lite does not have these restrictions.)
  • See the CSV section for information about ASV and USV.
  • JSON: ignores all separator flags from the command line.
  • Headerless CSV overlaps quite a bit with NIDX format using comma for IFS. See also the page on CSV with and without headers.
  • For XTAB, the record separator is a repetition of the field separator. For example, if one record has x=1,y=2 and the next has x=3,y=4, and OFS is newline, then output lines are x 1, then y 2, then an extra newline, then x 3, then y 4. This means: to customize XTAB, set OFS rather than ORS.
CSV Always \n; not alterable * Default ,; must be single-character None
TSV Always \n; not alterable * Default \t; must be single-character None
CSV-lite Default \n * Default , None
TSV-lite Default \n * Default \t None
JSON N/A; records are between { and } Always ,; not alterable Always :; not alterable
DKVP Default \n Default , Default =
NIDX Default \n Default space None
XTAB Not used; records are separated by an extra FS \n * Default: space with repeats
PPRINT Default \n * Space with repeats None
Markdown Always \n; not alterable * One or more spaces, then |, then one or more spaces; not alterable None

* or \r\n on Windows