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I'm passing a variable to a script on the command line. What is the character limit of a command? eg:

$ MyScript reallyreallyreally...reallyreallyreallylongoption

Thanks.

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The shell/OS imposed limit is 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 and 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.

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    On my system, getconf ARG_MAX gives 2097152, but the max arg length I can pass is still 131071 (and I don't have to deduct the size of the environment).
    – Dennis Williamson
    Jul 23, 2010 at 14:10
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    Also remember that xargs and even find -exec are your friends when dealing with giant argument lists.
    – coredump
    Jul 23, 2010 at 14:15
  • @Dennis: the value returned by getconf is kernel level I think. Perhaps bash is setting a lower limit by its design/configuration? Also, my knowledge of this comes from some time ago so it could be that things have changed a little recently, though it isn't an area I'd expect to see a lot of movement in except in new experimental shells.
    – David Spillett
    Jul 23, 2010 at 19:07
  • I get the same results in ksh, zsh, dash, fish and Bash 3 as I did in Bash 4. The error message from fish may be informative: "fish: The total size of the argument and environment lists (130kB) exceeds the operating system limit of 2.0MB." However, set | wc -c is 306317 and env | wc -c is 2507 which don't account for the difference. I don't know what else is being counted.
    – Dennis Williamson
    Jul 23, 2010 at 22:02
  • Is the call itself (to the path of the script/binary) considered as/in an argument? This would shorten the argument length if the target (path) is deeply located.
    – Ben
    Aug 19, 2021 at 13:06
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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).

See https://unix.stackexchange.com/questions/120642/what-defines-the-maximum-size-for-a-command-single-argument

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  • I think MAX_ARG_STRLEN has been based on current stack size limits since around 2011. Try ulimit -S -s unlimited before trying to execute a very long command line. Note that sudo will overwrite your limits so you cannot run very long sudo ... even after setting stack size.
    – Mikko Rantalainen
    Nov 2, 2021 at 11:03
  • @MikkoRantalainen, I can still see MAX_ARG_STRLEN being a hard-coded constant in latest Linux. Maybe you're thinking ARG_MAX?
    – Rotsor
    Jan 1, 2022 at 5:10
  • Yeah, I meant ARG_MAX because it defines the command line and size of the environment and scales automatically according to current stack. The MAX_ARG_STRLEN is hardcoded as you wrote and doesn't usually cause problems.
    – Mikko Rantalainen
    Jan 2, 2022 at 21:08
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    @MikkoRantalainen, except when it does, as is the case in this question.
    – Rotsor
    Jan 4, 2022 at 20:16
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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 $VAR
bash: /bin/bash: Argument list too long
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    Actually that's a very long variable value.
    – Dennis Williamson
    Jul 23, 2010 at 14:02
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    Also, you don't have to use perl and a script: /bin/echo "$(printf "%*s" 131071 ".")">/dev/null
    – Dennis Williamson
    Jul 23, 2010 at 14:13
  • @DennisWilliamson however, printf '%s\n' "$(printf '%*s' 131072 .)" >/dev/null works.
    – jarno
    Sep 24, 2017 at 5:24
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    That's because printf is a shell built-in, so bash does not need to do an exec() for spawning another process. ARG_MAX only matters to the length of the argument list of the exec functions (exec(), execl(), execlp(), execvp(), execvpe(), etc.).
    – Vicente Olivert Riera
    Jul 3, 2019 at 10:04
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xargs provides another method to get detailed information on command line length limit, including the theoretical maximum for your system, the usable maximum, the minimum specified by POSIX and the current buffer size limit.

You do this by providing no input to xargs and providing the option --show-limits. The man page also recommends providing the --no-run-if-empty option (a GNU extension).

Example Use/Output

$ xargs --no-run-if-empty --show-limits </dev/null
Your environment variables take up 3643 bytes
POSIX upper limit on argument length (this system): 2091461
POSIX smallest allowable upper limit on argument length (all systems): 4096
Maximum length of command we could actually use: 2087818
Size of command buffer we are actually using: 131072
Maximum parallelism (--max-procs must be no greater): 2147483647
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