LiveFire Labs' UNIX Tip, Trick, or Shell Script of the Week

Useful Shell Script Variables - Part V - SECONDS

Like last week's shell script variable RANDOM, this week's variable also provides a different value each time it's referenced.  The value stored in SECONDS indicates the number of seconds since the shell was invoked:

$ print $SECONDS; sleep 30; print $SECONDS
8
38
$

If a value is assigned to SECONDS, the sum of that value and the number of seconds since its assignment is displayed when the variable is referenced:

$ SECONDS=10; print $SECONDS; sleep 30; print $SECONDS
10
40
$

One possible use for this variable is to time how long a particular operation within your shell script takes to complete.  For example, you may want to know how long it takes to process a unique set of data during each pass through a loop construct.  The following  script illustrates this concept:

#!/bin/ksh

FILELIST="file1 file2 file3"

sleeptime=5

for i in ${FILELIST}
do
  # record the value of SECONDS before processing begins
  start=$SECONDS

  print "Processing $i..."

  # Normally, processing statements would replace
  # the following sleep statement
  sleep ${sleeptime}

  # record the value of SECONDS after processing ends
  stop=$SECONDS

  # calculate processing time
  (( runtime=stop-start ))
  print "Processing time for $i was $runtime seconds.\n"

  # increase the sleep time for next iteration of the loop
  (( sleeptime=sleeptime*2 ))

done

exit 0

Running this script produces the following output:

$ ./seconds
Processing file1...
Processing time for file1 was 5 seconds.

Processing file2...
Processing time for file2 was 10 seconds.

Processing file3...
Processing time for file3 was 20 seconds.

$

To provide a unique "runtime" value for each time through the loop in this script, the amount of time to sleep is doubled at the end of each iteration.  In an actual script, the counter and sleep statement may or may not be needed.

After the runtime is calculated, arithmetic statements can be added to convert the seconds to minutes if desired.