hash man page
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.
hash — remember or report utility locations
hash [utility...] hash -r
The hash utility shall affect the way the current shell environment remembers the locations of utilities found as described in Section 220.127.116.11, Command Search and Execution. Depending on the arguments specified, it shall add utility locations to its list of remembered locations or it shall purge the contents of the list. When no arguments are specified, it shall report on the contents of the list.
Utilities provided as built-ins to the shell shall not be reported by hash.
The hash utility shall conform to the Base Definitions volume of POSIX.1-2008, Section 12.2, Utility Syntax Guidelines.
The following option shall be supported:
Forget all previously remembered utility locations.
The following operand shall be supported:
The name of a utility to be searched for and added to the list of remembered locations. If utility contains one or more <slash> characters, the results are unspecified.
The following environment variables shall affect the execution of hash:
Provide a default value for the internationalization variables that are unset or null. (See the Base Definitions volume of POSIX.1-2008, Section 8.2, Internationalization Variables 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 message catalogs for the processing of LC_MESSAGES.
Determine the location of utility, as described in the Base Definitions volume of POSIX.1-2008, Chapter 8, Environment Variables.
The standard output of hash shall be used when no arguments are specified. Its format is unspecified, but includes the pathname of each utility in the list of remembered locations for the current shell environment. This list shall consist of those utilities named in previous hash invocations that have been invoked, and may contain those invoked and found through the normal command search process.
The standard error shall be used only for diagnostic messages.
The following exit values shall be returned:
An error occurred.
Consequences of Errors
The following sections are informative.
Since hash affects the current shell execution environment, it is always provided as a shell regular built-in. If it is called in a separate utility execution environment, such as one of the following:
nohup hash -r find . -type f | xargs hash
it does not affect the command search process of the caller's environment.
The hash utility may be implemented as an alias—for example, alias -t -, in which case utilities found through normal command search are not listed by the hash command.
The effects of hash -r can also be achieved portably by resetting the value of PATH; in the simplest form, this can be:
The use of hash with utility names is unnecessary for most applications, but may provide a performance improvement on a few implementations; normally, the hashing process is included by default.
Section 18.104.22.168, Command Search and Execution
The Base Definitions volume of POSIX.1-2008, Chapter 8, Environment Variables, Section 12.2, Utility Syntax Guidelines
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.unix.org/online.html .
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