How to Cat and Follow a File

catfilesopen filespipetail

A file is being sequentially downloaded by wget.

If I start unpacking it with cat myfile.tar.bz2 | tar -xj, it may unpack correctly or fail with "Unexpected EOF", depending on what is faster.

How to "cat and follow" a file, i.e. output content of the file to stdout, but don't exit on EOF, instead keep subsribed to that file and continue outputting new portions of the data, exiting only if the file is closed by writer and not re-opened within N seconds.

I've created a script cat_and_follow based on @arielCo's answer that also terminates the tail when the file is not being opened for writing anymore.

Best Answer

tail +1f file

I tested it on Ubuntu with the LibreOffice source tarball while wget was downloading it:

tail +1f libreoffice-4.2.5.2.tar.xz | tar -tvJf -

It also works on Solaris 10, RHEL3, AIX 5 and Busybox 1.22.1 in my Android phone (use tail +1 -f file with Busybox).

Related Solutions

Command Monitoring – How to Follow a Command Like tail -f

Try the watch command, although I suspect just about everyone has written their own version at one time or another. (The cheapie version is while :; do clear; "$@"; sleep 5; done.)

Follow Pipe with less – How to Follow a Pipe Using less

Pressing F or G makes less try to reach input EOF. If the input is a pipe, less hangs until the pipe is closed on the other side (and not "does nothing").

This can be worked around by saving the command output to a temporary file in the background, and then by using it as input for less:

command > /tmp/x &
less +F /tmp/x; kill %; rm /tmp/x

There is no option to do this in less only; however, I admit it would be useful.

Best Answer

Related Solutions

Command Monitoring – How to Follow a Command Like tail -f

Follow Pipe with less – How to Follow a Pipe Using less

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