How to add a timestamp to bash script log?

You can pipe the script's output through a loop that prefixes the current date and time:

./script.sh | while IFS= read -r line; do printf '%s %s\n' "$(date)" "$line"; done >>/var/log/logfile

If you'll be using this a lot, it's easy to make a bash function to handle the loop:

adddate() {
    while IFS= read -r line; do
        printf '%s %s\n' "$(date)" "$line";
    done
}

./thisscript.sh | adddate >>/var/log/logfile
./thatscript.sh | adddate >>/var/log/logfile
./theotherscript.sh | adddate >>/var/log/logfile

See ts from the Ubuntu moreutils package:

command | ts

Or, if $command does automatic buffering (requires expect-dev package):

unbuffer command | ts

The date command will provide that information

date -u
Sat Sep 10 22:39:24 UTC 2011

so you can

echo $(date -u) "Some message or other"

is that what you wanted ?

Make a config.sh file

#!/usr/bin/env bash
LOGFILE="/path/to/log.log"
TIMESTAMP=`date "+%Y-%m-%d %H:%M:%S"`

When you need to send to log file use

#!/usr/bin/env bash
source /path/to/config.sh

echo "$TIMESTAMP Say what you are doing" >> $LOGFILE

do_what_you_want >> $LOGFILE

Log file will looks like

2013-02-03 18:22:30 Say what you are doing

So it will be easy to sort by date

You can simply echo the command outputs to the logfile. ie,

echo "`date -u` `./script.sh`" >> /var/log/logfile

It really works :)

Example:

[sparx@E1]$ ./script.sh 
Hello Worldy
[sparx@E1]$ echo "`date -u` `./script.sh`" >> logfile.txt
[sparx@E1]$ cat logfile.txt 
Mon Sep 12 20:18:28 UTC 2011 Hello Worldy
[sparx@E1]$ 

The accepted answer https://serverfault.com/a/310104 can be a bit slow, if a lot of lines have to be processed, with the overhead of starting the date process allowing about 50 lines per second in Ubuntu, and only about 10-20 in Cygwin.

When bash can be assumed a faster alternative would be the printf builtin with its %(...)T format specifier. Compare

>> while true; do date; done | uniq -c
     47 Wed Nov  9 23:17:18 STD 2016
     56 Wed Nov  9 23:17:19 STD 2016
     55 Wed Nov  9 23:17:20 STD 2016
     51 Wed Nov  9 23:17:21 STD 2016
     50 Wed Nov  9 23:17:22 STD 2016

>> while true; do printf '%(%F %T)T\n'; done | uniq -c
  20300 2016-11-09 23:17:56
  31767 2016-11-09 23:17:57
  32109 2016-11-09 23:17:58
  31036 2016-11-09 23:17:59
  30714 2016-11-09 23:18:00

Remark. On my current OpenSuse work PC (July 2021) performance is significantly up, doing 1200 lines per second with date and 265,000 lines per second with printf. Not entirely clear, how this particular performance jumped by almost two orders of magnitude compared to my laptop from 5 years ago.

Try this

timestamp()
{
 date +"%Y-%m-%d %T"
}

Call this timestamp function in every echo command:

echo "$(timestamp): write your log here" >> /var/log/<logfile>.log

Short answer formatted to fit the question

script.sh | gawk '{ print strftime("[%Y-%m-%d %H:%M:%S]"), $0 }' >> /var/log/logfile

Explanation

awk runs fast and is able to work as Unix pipe filter and print date by itself.

gawk '{ print strftime("[%Y-%m-%d %H:%M:%S]"), $0 }'

Let's benchmark it:

yes |head -5000000 |gawk '{ print strftime("[%Y-%m-%d %H:%M:%S]"), $0 }' |uniq -c
 461592 [2017-02-28 19:46:44] y
 488555 [2017-02-28 19:46:45] y
 491205 [2017-02-28 19:46:46] y
 498568 [2017-02-28 19:46:47] y
 502605 [2017-02-28 19:46:48] y
 494048 [2017-02-28 19:46:49] y
 493299 [2017-02-28 19:46:50] y
 498005 [2017-02-28 19:46:51] y
 502916 [2017-02-28 19:46:52] y
 495550 [2017-02-28 19:46:53] y
  73657 [2017-02-28 19:46:54] y

Additional information

Sed appears to runs much faster,

sed -e "s/^/$(date -R) /"

yes |head -5000000 |sed -e "s/^/$(date -R) /" |uniq -c
5000000 Tue, 28 Feb 2017 19:57:00 -0500 y

However, on closer inspection, set does not seem to change time,

vmstat 1 | sed -e "s/^/$(date -R) /"

Because date (which is slower by the way) gets called only once.

You mean like:

(date && script.sh) >> /var/log/logfile

Another option is to setup a function to call each time you want to output data in your code:

PrintLog(){
  information=$1
  logFile=$2
  #echo "$(date +'%Y-%m-%d %H:%M:%S)'" ${information} >> $logFile
  #Improve performance by using printf instead of date
  echo "$(printf '%(%F %T)T')" ${1} >> ${logFile}
}

Then every time in your code you want to send it to the log file call

PrintLog "Stuff you want to add..." ${LogFileVariable}

Easy peasy....