Print the first or last lines of a file, follow changes, and optionally filter the output, replacing or highlighting text parts with the free sfk for windows and linux.

- download the free swiss file knife base from sourceforge.
- unpack the tool using winzip or unzip.
- open the Windows Command Line or a Linux shell.

sfk head [-lines=n] [-f[ollow] [filename]
sfk tail [-lines=n] [-f[ollow] [filename]
sfk ... +[t]head | +[t]tail [-lines=n]

1. print first or last lines of a file, optionally following changes.
   to use file content processing, provide a single filename.

2. print first or last text lines procuded by a previous command.
   to process chain text, thead or ttail are recommended.

options
   -lines=n     print first or last n lines (default is 10).
   -follow      or -f waits for file changes, printing them endlessly.
                if file is recreated or shrunk, rereads the last lines.
                if file is changed, head will always reread from front.
                to post-process tail output, e.g. with +filter, always
                add +loop at the end of the command sequence.
   -quiet       do not tell verbosely about read restarts.
   -polltime=n  with -follow, specifies the delay in milliseconds before
                the file is checked again for changes. default is 500.
   -altsize     use a different method to determine the file size
                (stat instead of seek). may help if the default method
                fails to read the file, or to improve performance.

see also
   sfk view     GUI tool to view all text files of a folder, then jump
                through file heads/ends by ctrl+pageDown or ctrl+end.

examples
   sfk tail -follow logs\access.log
      immediately lists last lines, then all added lines over time.
   sfk tail -f c:\temp\log.txt +filter -+error: -+warning: +loop
      endless filter of error and warning messages from log.txt.
   type myfile.txt | sfk filter +thead -lines=50
      filter first 50 lines from stdin, via sfk filter.

example sfk script with conditional execution
  file logfilter.txt:
    sfk label checklog
       +tail -follow logfile.txt
       +tee toterm +filter -+error:
       +if "rc>0" call myalert
       +loop +end
    sfk label myalert
       +run -yes "myalert.bat" +end
  sfk script logfilter.txt
     will run myalert.bat whenever errors appear in logfile.txt.
 
scripted example

if you want to both tail and filter on several phrases,
the command may become hard to read and to edit. as an
alternative, you may put it into a batch embedded sfk script,
e.g. under windows, create a file

   filtlog.bat:
      
      @echo off
      sfk script filtlog.bat -from begin
      goto xend
      
      sfk label begin
      
         +tail -f log.txt
      
         +filter
      
            -high red      error:*
            -high yellow   warning:*
            -high blue     debug:*
      
         +loop
      
      :xend
      
then type "filtlog". creating command sequences this way
allows better readability, and easy editing in any editor.

see also

sfk filter   - all options for selection and post-processing of text
sfk partcopy - copy byte block from the beginning of a file
sfk script   - how to create script files with sfk commands
sfk samp     - ready-to-use templates for windows and linux scripts

read more about all sfk functions here.