The General Way
$ cat input.log | sed -e "s/^/$(date -R) /" >> output.log
How it works:
cat reads file called
input.log and just prints it to its standard output stream.
Normally the standard output is connected to a terminal, but this little script contains
| so shell redirects the standard output of
cat to standard input of
sed reads data (as
cat produces it), processes it (according to the script provided with
-e option) and then prints it to its standard output. The script
"s/^/$(date -R) /" means replace every start of line to a text generated by
date -R command (the general construction for replace command is:
Then according to
bash redirects the output of
sed to a file called
> means replace file contents and
>> means append to the end).
The problem is the
$(date -R) evaluated once when you run the script so it will insert current timestamp to the beginning of each line. The current timestamp may be far from a moment when a message was generated. To avoid it you have to process messages as they are written to the file, not with a cron job.
The standard stream redirection described above called pipe. You can redirect it not just with
| between commands in the script, but through a FIFO file (aka named pipe). One program will write to the file and another will read data and receive it as the first sends.
Pick an example:
$ mkfifo foo.log.fifo
$ while true; do cat foo.log.fifo | sed -e "s/^/$(date -R) /" >> foo.log; done;
# have to open a second terminal at this point
$ echo "foo" > foo.log.fifo
$ echo "bar" > foo.log.fifo
$ echo "baz" > foo.log.fifo
$ cat foo.log
Tue, 20 Nov 2012 15:32:56 +0400 foo
Tue, 20 Nov 2012 15:33:27 +0400 bar
Tue, 20 Nov 2012 15:33:30 +0400 baz
How it works:
mkfifo creates a named pipe
while true; do sed ... ; done runs an infinite loop and at every iteration it runs
sed with redirecting
foo.log.fifo to its standard input;
sed blocks in waiting for input data and then processes a received message and prints it to standard output redirected to
At this point you have to open a new terminal window because the loop occupies the current terminal.
echo ... > foo.log.fifo prints a message to its standard output redirected to the fifo file and
sed receives it and processes and writes to a regular file.
The important note is the fifo just as any other pipe has no sense if one of its sides is not connected to any process. If you try to write to a pipe the current process will block until someone would read data on the other side of the pipe. If you want to read from a pipe the process will block until someone will write data to the pipe. The
sed loop in the example above does nothing (sleeps) until you do
For your particular situation you just configure your application to write log messages to the fifo file. If you can't configure it - simply delete the original log file and create a fifo file. But note again that if the
sed loop will die for some reason - your program will be blocked upon attempting to
write to the file until someone will
read from the fifo.
The benefit is the current timestamp evaluated and attached to a message as the program writes it to the file.
Asynchronous Processing With
To make writing to the log and processing more independent you can use two regular files with
tailf. An application will write message to a raw file and other process read new lines (follow to writes asynchronously) and process data with writing to the second file.
Let's take an example:
# will occupy current shell
$ tailf -n0 bar.raw.log | while read line; do echo "$(date -R) $line" >> bar.log; done;
$ echo "foo" >> bar.raw.log
$ echo "bar" >> bar.raw.log
$ echo "baz" >> bar.raw.log
$ cat bar.log
Wed, 21 Nov 2012 16:15:33 +0400 foo
Wed, 21 Nov 2012 16:15:36 +0400 bar
Wed, 21 Nov 2012 16:15:39 +0400 baz
How it works:
tailf process that will follow writes to
bar.raw.log and print them to standard output redirected to the infinite
while read ... echo loop. This loop performs two actions: read data from standard input to a buffer variable called
line and then write generated timestamp with the following buffered data to the
Write some messages to the
bar.raw.log. You have to do this in a separate terminal window because the first one will be occupied by
tailf which will follow the writes and do its job. Quite simple.
The pros is your application would not block if you kill
tailf. The cons is less accurate timestamps and duplicating log files.