Bash – How to limit the number of lines a command’s output has available in bash

bashnohupstdouttail

I started downloading a big file in the background using

$ nohup wget http://example.tld/big.iso &

which also gives me a nohup.out file that includes the output of wget.

Now, if I later want to watch the downloading process, I could use $ tail -f nohup.out but that fills up my terminal window faster than I'd wish for. What I'd like to see is the last line constantly updating (just like when using wget alone).

I tried $ tail -n 1 -f nohup.out but it seems to affect only the initial tailin'.

Generally speaking, if it is possible to limit (in this case to 1) the number of lines a command's output has available/visible it would solve this problem. Sort of having the output in a Circular buffer — just think of the normal progress bar $ wget example.tld/big.iso would print.

Is there such a solution?

Or am I climbing the tree wrong way? (Meaning, would it be easier to limit nohup's output or do something else?)

Best Answer

If you do not want to limit the scrolling region (see my other answer), you can also use the carriage return to go back to the beginning of the line before printing the next line. There is an escape sequence that clears the rest of the line, which is necessary when the current line is shorter than the previous line.

nohup wget http://cdimage.debian.org/debian-cd/6.0.3/amd64/iso-dvd/debian-6.0.3-amd64-DVD-1.iso &
el="$(tput el)"; # Clear to the end of the line
tail -n 1 -f nohup.out | while read -r line; do echo -n $'\r'"$el$line"; done;

Related Solutions

Command Line – How Does ‘tail -f’ Parameter Work

From the tail(1) man page:

   With  --follow  (-f),  tail  defaults to following the file descriptor,
   which means that even if a tail’ed file is renamed, tail will  continue
   to  track  its  end.   This  default behavior is not desirable when you
   really want to track the actual name of the file, not the file descrip-
   tor (e.g., log rotation).  Use --follow=name in that case.  That causes
   tail to track the named file  in  a  way  that  accommodates  renaming,
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Your text editor is renaming or deleting the original file and saving the new file under the same filename. Use -F instead.

IO – How to Limit Output Speed of stdout

You can add a rate limiting tool to your pipeline.

For example there is pv which has a rate-limiting option:

-L RATE, --rate-limit RATE
Limit the transfer to a maximum of RATE bytes per second. A suffix of "k", "m", "g", or "t" can be added to denote kilobytes (*1024), megabytes, and so on

An alternative is the tool buffer which has:

   -u microseconds
After every write pause for this many microseconds. Defaults to zero. (Surprisingly a small sleep, 100 usecs, after each write can greatly enhance throughput on some drives.)

Best Answer

Related Solutions

Command Line – How Does ‘tail -f’ Parameter Work

IO – How to Limit Output Speed of stdout

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