How long can the command line be?

This page attempts to explain the meaning and the consequences of the ARG_MAX kernel constant. It's more or less related to my Useless Use of X Award page and a recurring topic in comp.unix.shell

The ARG_MAX constant
Consequences
Further reading

The ARG_MAX constant

The value of the constant ARG_MAX defines how long an argument list your kernel can take on the command line before it chokes. Here's a script to find out the value of it:

	# Print out the system's architecture
	uname -rms              # or uname -a

	# Let the C preprocessor read in the system's limits from limits.h 
	# and then expand the macro ARG_MAX
	# Filter out excess output (there will be some line number information
	# and a gadzillion empty lines)
	#
	# On some gcc systems, I have been unable to find a working cpp.
	# You can use gcc -E - <:<:HERE | tail -1 in that case.

	cpp <<HERE | tail -1    # Only the last line is interesting
	#include <limits.h>
	ARG_MAX
	HERE

And here are some results from various machines I happen to have access to:

```
	Linux 2.0.33 i686
	131072
```
```
	SunOS 5.4 sun4m
	1048320
```
```
	SunOS 4.1.3 sun4c
	ARG_MAX
```
Curiously, SunOS 4 knows about _POSIX_ARG_MAX, which is correctly set to 4096.
According to Stevens, SunOS 4.1.1 allows a megabyte of information to be passed (1048576 bytes).
```
	OSF1 V4.0 alpha
	38912
```
```
	IRIX 5.3 IP12
	ARG_MAX
```
There is something funny about how IRIX does these things but I found the value 5120 in an #ifndef in limits.h
```
	NetBSD 1.2C mac68k
	(256 * 1024)
```
(For the lazy people without dc(1) handy, that's 262144 bytes.)

POSIX requires ARG_MAX to be at least 4096 bytes. There are allegedly old systems with ARG_MAX set as low as 2048 or even 1024 bytes.

Consequences

On a system with a low value for ARG_MAX, you will often end up with the shell saying Too many arguments or Argument list too long when you attempt something like

	ls * */*

or even

	ls *

and of course

	for f in `find . -print`; do
		: something
	done

On any system, the last of the examples is going to blow up sooner or later. This is why we have xargs:

	find . -print |
	xargs : something

This is of course still not a secure construction (depending of course on what the : something attempts to do). But that's another story.

How long can the command line be?

Contents

The ARG_MAX constant

Consequences

Further reading