# Running shell commands

TODO: while-read example from issues

The [system](reference-dsl.md#system) DSL function allows you to run a specific shell command and put its output -- minus the final newline -- into a record field. The command itself is any string, either a literal string, or a concatenation of strings, perhaps including other field values or what have you.

GENMD-RUN-COMMAND
mlr --opprint put '$o = system("echo hello world")' data/small
GENMD-EOF

GENMD-RUN-COMMAND
mlr --opprint put '$o = system("echo {" . NR . "}")' data/small
GENMD-EOF

GENMD-RUN-COMMAND
mlr --opprint put '$o = system("echo -n ".$a."| md5")' data/small
GENMD-EOF

Note that running a subprocess on every record takes a non-trivial amount of time. Comparing asking the system `date` command for the current time in nanoseconds versus computing it in process:

<!--- hard-coded, not live-code, since %N doesn't exist on all platforms -->

GENMD-CARDIFY-HIGHLIGHT-ONE
mlr --opprint put '$t=system("date +%s.%N")' then step -a delta -f t data/small
a   b   i x                   y                   t                    t_delta
pan pan 1 0.3467901443380824  0.7268028627434533  1568774318.513903817 0
eks pan 2 0.7586799647899636  0.5221511083334797  1568774318.514722876 0.000819
wye wye 3 0.20460330576630303 0.33831852551664776 1568774318.515618046 0.000895
eks wye 4 0.38139939387114097 0.13418874328430463 1568774318.516547441 0.000929
wye pan 5 0.5732889198020006  0.8636244699032729  1568774318.517518828 0.000971
GENMD-EOF

GENMD-CARDIFY-HIGHLIGHT-ONE
mlr --opprint put '$t=systime()' then step -a delta -f t data/small
a   b   i x                   y                   t                 t_delta
pan pan 1 0.3467901443380824  0.7268028627434533  1568774318.518699 0
eks pan 2 0.7586799647899636  0.5221511083334797  1568774318.518717 0.000018
wye wye 3 0.20460330576630303 0.33831852551664776 1568774318.518723 0.000006
eks wye 4 0.38139939387114097 0.13418874328430463 1568774318.518727 0.000004
wye pan 5 0.5732889198020006  0.8636244699032729  1568774318.518730 0.000003
GENMD-EOF
