tail: Output the last part of files
tail prints the last part (10 lines by default) of each
file; it reads from standard input if no files are given or
when given a file of ‘-’. Synopsis:
tail [option]… [file]…
If more than one file is specified,
tail prints a
one-line header consisting of:
==> file name <==
before the output for each file.
tail can output any amount of data (some other versions of
tail cannot). It also has no -r option (print in
reverse), since reversing a file is really a different job from printing
the end of a file; BSD
tail (which is the one with -r) can
only reverse files that are at most as large as its buffer, which is
typically 32 KiB. A more reliable and versatile way to reverse files is
The program accepts the following options. Also see Common options.
Output the last k bytes, instead of final lines. However, if k starts with a ‘+’, start printing with the kth byte from the start of each file, instead of from the end. k may be, or may be an integer optionally followed by, one of the following multiplicative suffixes:
‘b’ => 512 ("blocks") ‘KB’ => 1000 (KiloBytes) ‘K’ => 1024 (KibiBytes) ‘MB’ => 1000*1000 (MegaBytes) ‘M’ => 1024*1024 (MebiBytes) ‘GB’ => 1000*1000*1000 (GigaBytes) ‘G’ => 1024*1024*1024 (GibiBytes)
and so on for ‘T’, ‘P’, ‘E’, ‘Z’, and ‘Y’.
Loop forever trying to read more characters at the end of the file,
presumably because the file is growing.
If more than one file is given,
tail prints a header whenever it
gets output from a different file, to indicate which file that output is
There are two ways to specify how you’d like to track files with this option, but that difference is noticeable only when a followed file is removed or renamed. If you’d like to continue to track the end of a growing file even after it has been unlinked, use --follow=descriptor. This is the default behavior, but it is not useful if you’re tracking a log file that may be rotated (removed or renamed, then reopened). In that case, use --follow=name to track the named file, perhaps by reopening it periodically to see if it has been removed and recreated by some other program. Note that the inotify-based implementation handles this case without the need for any periodic reopening.
No matter which method you use, if the tracked file is determined to have
tail prints a message saying the file has been truncated
and resumes tracking the end of the file from the newly-determined endpoint.
When a file is removed,
tail’s behavior depends on whether it is
following the name or the descriptor. When following by name, tail can
detect that a file has been removed and gives a message to that effect,
and if --retry has been specified it will continue checking
periodically to see if the file reappears.
When following a descriptor, tail does not detect that the file has
been unlinked or renamed and issues no message; even though the file
may no longer be accessible via its original name, it may still be
The option values ‘descriptor’ and ‘name’ may be specified only with the long form of the option, not with -f.
The -f option is ignored if no file operand is specified and standard input is a FIFO or a pipe. Likewise, the -f option has no effect for any operand specified as ‘-’, when standard input is a FIFO or a pipe.
With kernel inotify support, output is triggered by file changes
and is generally very prompt.
tail sleeps for one second between checks—
use --sleep-interval=n to change that default—which can
make the output appear slightly less responsive or bursty.
When using tail without inotify support, you can make it more responsive
by using a sub-second sleep interval, e.g., via an alias like this:
alias tail='tail -s.1'
This option is the same as --follow=name --retry. That is, tail will attempt to reopen a file when it is removed. Should this fail, tail will keep trying until it becomes accessible again.
Indefinitely try to open the specified file. This option is useful mainly when following (and otherwise issues a warning).
When following by file descriptor (i.e., with --follow=descriptor),
this option only affects the initial open of the file, as after a successful
tail will start following the file descriptor.
When following by name (i.e., with --follow=name),
infinitely retries to re-open the given files until killed.
Without this option, when
tail encounters a file that doesn’t
exist or is otherwise inaccessible, it reports that fact and
never checks it again.
Change the number of seconds to wait between iterations (the default is 1.0).
During one iteration, every specified file is checked to see if it has
Historical implementations of
tail have required that
number be an integer. However, GNU
an arbitrary floating point number. See Floating point.
tail uses inotify, this polling-related option
is usually ignored. However, if you also specify --pid=p,
tail checks whether process p is alive at least
every number seconds.
When following by name or by descriptor, you may specify the process ID,
pid, of the sole writer of all file arguments. Then, shortly
after that process terminates, tail will also terminate. This will
work properly only if the writer and the tailing process are running on
the same machine. For example, to save the output of a build in a file
and to watch the file grow, if you invoke
like this then the tail process will stop when your build completes.
Without this option, you would have had to kill the
$ make >& makerr & tail --pid=$! -f makerr
If you specify a pid that is not in use or that does not correspond
to the process that is writing to the tailed files, then
may terminate long before any files stop growing or it may not
terminate until long after the real writer has terminated.
Note that --pid cannot be supported on some systems;
will print a warning if this is the case.
When tailing a file by name, if there have been n (default
iterations for which the file has not changed, then
fstat the file to determine if that file name is
still associated with the same device/inode-number pair as before.
When following a log file that is rotated, this is approximately the
number of seconds between when tail prints the last pre-rotation lines
and when it prints the lines that have accumulated in the new log file.
This option is meaningful only when polling (i.e., without inotify)
and when following by name.
Output the last k lines. However, if k starts with a ‘+’, start printing with the kth line from the start of each file, instead of from the end. Size multiplier suffixes are the same as with the -c option.
Never print file name headers.
Always print file name headers.
tail also supports an obsolete usage
‘tail -[count][bcl][f] [file]’, which is recognized
only if it does not conflict with the usage described
above. This obsolete form uses exactly one option and at most one
file. In the option, count is an optional decimal number optionally
followed by a size letter (‘b’, ‘c’, ‘l’) to mean count
by 512-byte blocks, bytes, or lines, optionally followed by ‘f’
which has the same meaning as -f.
On older systems, the leading ‘-’ can be replaced by ‘+’ in
the obsolete option syntax with the same meaning as in counts, and
obsolete usage overrides normal usage when the two conflict.
This obsolete behavior can be enabled or disabled with the
_POSIX2_VERSION environment variable (see Standards conformance).
Scripts intended for use on standard hosts should avoid obsolete syntax and should use -c count[b], -n count, and/or -f instead. If your script must also run on hosts that support only the obsolete syntax, you can often rewrite it to avoid problematic usages, e.g., by using ‘sed -n '$p'’ rather than ‘tail -1’. If that’s not possible, the script can use a test like ‘if tail -c +1 </dev/null >/dev/null 2>&1; then …’ to decide which syntax to use.
Even if your script assumes the standard behavior, you should still beware usages whose behaviors differ depending on the POSIX version. For example, avoid ‘tail - main.c’, since it might be interpreted as either ‘tail main.c’ or as ‘tail -- - main.c’; avoid ‘tail -c 4’, since it might mean either ‘tail -c4’ or ‘tail -c 10 4’; and avoid ‘tail +4’, since it might mean either ‘tail ./+4’ or ‘tail -n +4’.
An exit status of zero indicates success, and a nonzero value indicates failure.