ULIMIT(1) General Commands Manual ULIMIT(1)

ulimit — report or set resource limits

ulimit [-H | -S] -a

ulimit [-H | -S] [-c | -d | -f | -n | -s | -t | -v] [newlimit]

The ulimit utility shall report or set the resource limits in effect in the process in which it is executed.

Soft limits can be changed by a process to any value that is less than or equal to the hard limit. A process can (irreversibly) lower its hard limit to any value that is greater than or equal to the soft limit. Only a process with appropriate privileges can raise a hard limit.

The value unlimited for a resource shall be considered to be larger than any other limit value. When a resource has this limit value, the implementation shall not enforce limits on that resource. In locales other than the POSIX locale, ulimit may support additional non-numeric values with the same meaning as unlimited.

The behavior when resource limits are exceeded shall be as described in the System Interfaces volume of POSIX.1-2024 for the setrlimit(3) function.

The ulimit utility shall conform to XBD V1_chap12(7), except that:

  • The order in which options other than -H, -S, and -a are specified may be significant.
  • Conforming applications shall specify each option separately; that is, grouping option letters (for example, -fH) need not be recognized by all implementations.

The following options shall be supported:

Report hard limit(s) or set only a hard limit.
Report soft limit(s) or set only a soft limit.
Report the limit value for all of the resources named below and for any implementation-specific additional resources.
Report, or set if the newlimit operand is present, the core image size limit(s) in units of 512 bytes. [RLIMIT_CORE]
Report, or set if the newlimit operand is present, the data segment size limit(s) in units of 1024 bytes. [RLIMIT_DATA]
Report, or set if the newlimit operand is present, the file size limit(s) in units of 512 bytes. [RLIMIT_FSIZE]
Report, or set if the newlimit operand is present, the limit(s) on the number of open file descriptors, given as a number one greater than the maximum value that the system assigns to a newly-created descriptor. [RLIMIT_NOFILE]
Report, or set if the newlimit operand is present, the stack size limit(s) in units of 1024 bytes. [RLIMIT_STACK]
[XSI] Report, or set if the newlimit operand is present, the per-process CPU time limit(s) in units of seconds. [RLIMIT_CPU]
Report, or set if the newlimit operand is present, the address space size limit(s) in units of 1024 bytes. [RLIMIT_AS]

Where an option description is followed by [RLIMIT_ ] it indicates which resource for the getrlimit(3) and setrlimit(3) functions, defined in the System Interfaces volume of POSIX.1-2024, the option corresponds to.

If neither the -H nor -S option is specified:

  • If the newlimit operand is present, it shall be used as the new value for both the hard and soft limits.
  • If the newlimit operand is not present, -S shall be the default.

If no options other than -H or -S are specified, the behavior shall be as if the -f option was (also) specified.

If any option other than -H or -S is repeated, the behavior is unspecified.

The following operand shall be supported:

newlimit
Either an integer value to use as the new limit(s) for the specified resource, in the units specified in OPTIONS, or a non-numeric string indicating no limit, as described in the DESCRIPTION section. Numerals in the range 0 to the maximum limit value supported by the implementation for any resource shall be syntactically recognized as numeric values.

Not used.

None.

The following environment variables shall affect the execution of ulimit:

Provide a default value for the internationalization variables that are unset or null. (See XBD for the precedence of internationalization variables used to determine the values of locale categories.)
If set to a non-empty string value, override the values of all the other internationalization variables.
Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single-byte as opposed to multi-byte characters in arguments).

Determine the locale that should be used to affect the format and contents of diagnostic messages written to standard error.
Determine the location of messages objects and message catalogs.

Default.

The standard output shall be used when no newlimit operand is present.

If the -a option is specified, the output written for each resource shall consist of one line that includes:

  • A short phrase identifying the resource (for example "file size").
  • An indication of the units used for the resource, if the corresponding option description in OPTIONS specifies the units to be used.
  • The ulimit option used to specify the resource.
  • The limit value.

The format used within each line is unspecified, except that the format used for the limit value shall be as described below for the case where a single limit value is written.

If a single limit value is to be written; that is, the -a option is not specified and at most one option other than -H or -S is specified:

  • If the resource being reported has a numeric limit, the limit value shall be written in the following format: 
    "%1d\n", <limit value>

    where < > is the value of the limit in the units specified in OPTIONS.

  • If the resource being reported does not have a numeric limit, in the POSIX locale the following format shall be used: 
    "unlimited\n"

The standard error shall be used only for diagnostic messages.

None.

None.

The following exit values shall be returned:

Successful completion.
A request for a higher limit was rejected or an error occurred.

Default.

This utility is required to be intrinsic. See 1.7 Intrinsic Utilities for details.

Since ulimit affects the current shell execution environment, it is always provided as a shell regular built-in. If it is called with an operand in a separate utility execution environment, such as one of the following:

nohup ulimit -f 10000
env ulimit -S -c 10000

it does not affect the limit(s) in the caller's environment.

See also the APPLICATION USAGE for getrlimit(3).

Set the hard and soft file size limits to 51200 bytes:

ulimit -f 100

Save and restore a soft resource limit (where is an option letter specifying a resource):

saved=$(ulimit -X)

...

ulimit -X -S "$saved"

Execute a utility with a CPU limit of 5 minutes (using an asynchronous subshell to ensure the limit is set in a child process):

(ulimit -t 300; exec utility_name </dev/null) &
wait $!

The ulimit utility has no equivalent of the special values RLIM_SAVED_MAX and RLIM_SAVED_CUR returned by getrlimit(3), as ulimit is required to be able to output, and accept as input, all numeric limit values supported by the system.

Implementations differ in their behavior when the -a option is not specified and more than one option other than -H or -S is specified. Some write output for all of the specified resources in the same format as for -a; others write only the value for the last specified option. Both behaviors are allowed by the standard, since the SYNOPSIS lists the options as mutually exclusive.

None.

XBD V1_chap08(7), V1_chap12(7)

XSH getrlimit(3)

First released in Issue 2.

SD5-XCU-ERN-97 is applied, updating the SYNOPSIS.

Austin Group Defect 854 is applied, adding a note to the APPLICATION USAGE section that this utility is required to be intrinsic.

Austin Group Defect 1122 is applied, changing the description of NLSPATH .

Austin Group Defect 1418 is applied, adding the -H, -S, -a, -c, -d, -n, -s, -t, and -v options, and relating the -f option to the RLIMIT_FSIZE resource for setrlimit(3).

Austin Group Defect 1669 is applied, moving the ulimit utility, excluding the -t option, from the XSI option to the Base.

June 14, 2024 posix.fail