Bash: infinite sleep (infinite blocking)

I use startx to start X which will evaluate my .xinitrc. In my .xinitrc I start my window manager using /usr/bin/mywm. Now, if I kill my WM (in order to f.e. test some other WM), X will terminate too because the .xinitrc script reached EOF. So I added this at the end of my .xinitrc:

while true; do sleep 10000; done

This way X won't terminate if I kill my WM. Now my question: how can I do an infinite sleep instead of looping sleep? Is there a command which will kinda like freeze the script?

Solution 1:

sleep infinity does exactly what it suggests and works without cat abuse.

Solution 2:

tail does not block

As always: For everything there is an answer which is short, easy to understand, easy to follow and completely wrong. Here tail -f /dev/null falls into this category ;)

If you look at it with strace tail -f /dev/null you will notice, that this solution is far from blocking! It's probably even worse than the sleep solution in the question, as it uses (under Linux) precious resources like the inotify system. Also other processes which write to /dev/null make tail loop. (On my Ubuntu64 16.10 this adds several 10 syscalls per second on an already busy system.)

The question was for a blocking command

Unfortunately, there is no such thing ..

Read: I do not know any way to archive this with the shell directly.

Everything (even sleep infinity) can be interrupted by some signal. So if you want to be really sure it does not exceptionally return, it must run in a loop, like you already did for your sleep. Please note, that (on Linux) /bin/sleep apparently is capped at 24 days (have a look at strace sleep infinity), hence the best you can do probably is:

while :; do sleep 2073600; done

(Note that I believe sleep loops internally for higher values than 24 days, but this means: It is not blocking, it is very slowly looping. So why not move this loop to the outside?)

.. but you can come quite near with an unnamed fifo

You can create something which really blocks as long as there are no signals send to the process. Following uses bash 4, 2 PIDs and 1 fifo:

bash -c 'coproc { exec >&-; read; }; eval exec "${COPROC[0]}<&-"; wait'

You can check that this really blocks with strace if you like:

strace -ff bash -c '..see above..'

How this was constructed

read blocks if there is no input data (see some other answers). However, the tty (aka. stdin) usually is not a good source, as it is closed when the user logs out. Also it might steal some input from the tty. Not nice.

To make read block, we need to wait for something like a fifo which will never return anything. In bash 4 there is a command which can exactly provide us with such a fifo: coproc. If we also wait the blocking read (which is our coproc), we are done. Sadly this needs to keep open two PIDs and a fifo.

Variant with a named fifo

If you do not bother using a named fifo, you can do this as follows:

mkfifo "$HOME/.pause.fifo" 2>/dev/null; read <"$HOME/.pause.fifo"

Not using a loop on the read is a bit sloppy, but you can reuse this fifo as often as you like and make the reads terminat using touch "$HOME/.pause.fifo" (if there are more than a single read waiting, all are terminated at once).

Or use the Linux pause() syscall

For the infinite blocking there is a Linux kernel call, called pause(), which does what we want: Wait forever (until a signal arrives). However there is no userspace program for this (yet).

C

Create such a program is easy. Here is a snippet to create a very small Linux program called pause which pauses indefinitely (needs diet, gcc etc.):

printf '#include <unistd.h>\nint main(){for(;;)pause();}' > pause.c;
diet -Os cc pause.c -o pause;
strip -s pause;
ls -al pause

python

If you do not want to compile something yourself, but you have python installed, you can use this under Linux:

python -c 'while 1: import ctypes; ctypes.CDLL(None).pause()'

(Note: Use exec python -c ... to replace the current shell, this frees one PID. The solution can be improved with some IO redirection as well, freeing unused FDs. This is up to you.)

How this works (I think): ctypes.CDLL(None) loads the standard C library and runs the pause() function in it within some additional loop. Less efficient than the C version, but works.

My recommendation for you:

Stay at the looping sleep. It's easy to understand, very portable, and blocks most of the time.

Solution 3:

Maybe this seems ugly, but why not just run cat and let it wait for input forever?

Solution 4:

TL;DR: sleep infinity actually sleeps the maximum time allowed, which is finite.

Wondering why this is not documented anywhere, I bothered to read the sources from GNU coreutils and I found it executes roughly what follows:

1. Use strtod from C stdlib on the first argument to convert 'infinity' to a double precision value. So, assuming IEEE 754 double precision the 64-bit positive infinity value is stored in the seconds variable.
2. Invoke xnanosleep(seconds) (found in gnulib), this in turn invokes dtotimespec(seconds) (also in gnulib) to convert from double to struct timespec.
3. struct timespec is just a pair of numbers: integer part (in seconds) and fractional part (in nanoseconds). Naïvely converting positive infinity to integer would result in undefined behaviour (see §6.3.1.4 from C standard), so instead it truncates to TYPE_MAXIMUM(time_t).
4. The actual value of TYPE_MAXIMUM(time_t) is not set in the standard (even sizeof(time_t) isn't); so, for the sake of example let's pick x86-64 from a recent Linux kernel.

This is TIME_T_MAX in the Linux kernel, which is defined (time.h) as:

(time_t)((1UL << ((sizeof(time_t) << 3) - 1)) - 1)

Note that time_t is __kernel_time_t and time_t is long; the LP64 data model is used, so sizeof(long) is 8 (64 bits).

Which results in: TIME_T_MAX = 9223372036854775807.

That is: sleep infinite results in an actual sleep time of 9223372036854775807 seconds (10^11 years). And for 32-bit linux systems (sizeof(long) is 4 (32 bits)): 2147483647 seconds (68 years; see also year 2038 problem).

Edit: apparently the nanoseconds function called is not directly the syscall, but an OS-dependent wrapper (also defined in gnulib).

There's an extra step as a result: for some systems where HAVE_BUG_BIG_NANOSLEEP is true the sleep is truncated to 24 days and then called in a loop. This is the case for some (or all?) Linux distros. Note that this wrapper may be not used if a configure-time test succeeds (source).

In particular, that would be 24 * 24 * 60 * 60 = 2073600 seconds (plus 999999999 nanoseconds); but this is called in a loop in order to respect the specified total sleep time. Therefore the previous conclusions remain valid.

In conclusion, the resulting sleep time is not infinite but high enough for all practical purposes, even if the resulting actual time lapse is not portable; that depends on the OS and architecture.

To answer the original question, this is obviously good enough but if for some reason (a very resource-constrained system) you really want to avoid an useless extra countdown timer, I guess the most correct alternative is to use the cat method described in other answers.

Edit: recent GNU coreutils versions will try to use the pause syscall (if available) instead of looping. The previous argument is no longer valid when targeting these newer versions in Linux (and possibly BSD).

Portability

This is an important valid concern:

• sleep infinity is a GNU coreutils extension not contemplated in POSIX. GNU's implementation also supports a "fancy" syntax for time durations, like sleep 1h 5.2s while POSIX only allows a positive integer (e.g. sleep 0.5 is not allowed).
• Some compatible implementations: GNU coreutils, FreeBSD (at least from version 8.2?), Busybox (requires to be compiled with options FANCY_SLEEP and FLOAT_DURATION).
• The strtod behaviour is C and POSIX compatible (i.e. strtod("infinity", 0) is always valid in C99-conformant implementations, see §7.20.1.3).

Solution 5:

sleep infinity looks most elegant, but sometimes it doesn't work for some reason. In that case, you can try other blocking commands such as cat, read, tail -f /dev/null, grep a etc.