bash tee script output to file and stdout does not update timestamp

Question

I have the following bash command to tee script output to stdout and logfile. It works and I can see the live output. However the date is not changing. Is there any way to instruct exec to evaluate date in a dynamic way.

#!/bin/bash

LOGFILE="/tmp/te/script.log"
exec 1> >( stdbuf -e 0 -o 0 sed "s/^/$(date '+[%F %T]'): /" | tee -a ${LOGFILE}) 2>&1

echo "Started"
sleep 2
echo "Done sleeping"
sleep 2
echo "Another sleep"
sleep 2

echo "Done"

Thanks

Completely to be expected. `sed` is started *once*, and `date` is run *once* -- you wouldn't want it to be any other way; invoking external tools is extremely slow, so the log process couldn't necessarily keep up if you had a fast run of content going through. — Charles Duffy, Sep 26 '17 at 16:07

score 4 · Answer 1 · answered Sep 26 '17 at 16:02

4

You might want to use ts to add timestamps instead. For Ubuntu, apt install moreutils

exec 1> >(stdbuf -e0 -o0 ts '[%F %T]' | tee -a "$LOGFILE") 2>&1
# ...................... ^^^^^^^^^^^^

answered Sep 26 '17 at 16:02

glenn jackman

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Charles Duffy · Answer 2 · 2017-09-26T16:06:08.710

If you have bash 4.1 or newer, it has a very efficient means of calculating timestamp via printf %()T. Thus:

logfile="/tmp/te/script.log"
exec > >(while IFS= read -r line; do
          printf '%([%F %T])T %s\n' -1 "$line"
         done >>"$logfile") 2>&1

This uses a BashFAQ #1 while read loop to iterate over lines, and uses printf to generate a line of output with the current timestamp for each.

_{(If you get the error printf: `(': invalid format character, that means your bash is too old; you could instead printf '%s %s\n' "$(date '+[%F %T]')" "$line", but this is going to be much slower).}

bash tee script output to file and stdout does not update timestamp

2 Answers2