I'm passing a variable to a script on the command line. What is the character limit of a command? eg:
$ MyScript reallyreallyreally...reallyreallyreallylongoption
The shell/OS imposed limit is generally very long - usually one or two hundred thousand characters.
getconf ARG_MAX will give you the maximum input limit for a command. On the Debian system I currently have a terminal open on this returns 131072 which is 128*1024. The limit is reduced by your environment variables as if my memory serves me correctly these are passed in the same structure by the shell, though that will only take off a few hundred characters in most cases. To find an approximation of this value run
env | wc -c - this suggests 325 characters at the current time on this login on this machine.
Scripts are likely to permit this full length, but it is not unlikely that other utilities will impose their own limits either intentionally or through design issues. There may also be artificial limits to how long an individual argument on a long command line can be, and/or how long a path to a file can be.
ARG_MAX indeed limits the total size of the command line and the environment, but you're facing an additional limitation: one argument must not be longer than MAX_ARG_STRLEN (which is unfortunately hard-coded to be 131072).
Do you mean what is the longest variable length? To figure that out you can use perl's "x" to create a very long variable name:
VAR=`perl -e 'print "a"x131071'` ; bash a.sh $VAR
On My system 131071 works:
and the variable is printed at 131072 it's too big:
VAR=`perl -e 'print "a"x131072'` ; bash a.sh $VAR bash: /bin/bash: Argument list too long