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.
basename string [suffix]
The string operand shall be treated as a pathname, as defined in the Base Definitions volume of POSIX.1-2017, Section 3.271, Pathname. The string string shall be converted to the filename corresponding to the last pathname component in string and then the suffix string suffix, if present, shall be removed. This shall be done by performing actions equivalent to the following steps in order:
- If string is a null string, it is unspecified whether the resulting string is '.' or a null string. In either case, skip steps 2 through 6.
- If string is "//", it is implementation-defined whether steps 3 to 6 are skipped or processed.
- If string consists entirely of <slash> characters, string shall be set to a single <slash> character. In this case, skip steps 4 to 6.
- If there are any trailing <slash> characters in string, they shall be removed.
- If there are any <slash> characters remaining in string, the prefix of string up to and including the last <slash> character in string shall be removed.
- If the suffix operand is present, is not identical to the characters remaining in string, and is identical to a suffix of the characters remaining in string, the suffix suffix shall be removed from string. Otherwise, string is not modified by this step. It shall not be considered an error if suffix is not found in string.
The resulting string shall be written to standard output.
The following operands shall be supported:
The following environment variables shall affect the execution of basename:
Provide a default value for the internationalization variables that are unset or null. (See the Base Definitions volume of POSIX.1-2017, 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.
The basename utility shall write a line to the standard output in the following format:
"%s\n", <resulting string>
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.
The definition of pathname specifies implementation-defined behavior for pathnames starting with two <slash> characters. Therefore, applications shall not arbitrarily add <slash> characters to the beginning of a pathname unless they can ensure that there are more or less than two or are prepared to deal with the implementation-defined consequences.
If the string string is a valid pathname:
$(basename -- "string")
produces a filename that could be used to open the file named by string in the directory returned by:
$(dirname -- "string")
If the string string is not a valid pathname, the same algorithm is used, but the result need not be a valid filename. The basename utility is not expected to make any judgements about the validity of string as a pathname; it just follows the specified algorithm to produce a result string.
The following shell script compiles /usr/src/cmd/cat.c and moves the output to a file named cat in the current directory when invoked with the argument /usr/src/cmd/cat or with the argument /usr/src/cmd/cat.c:
c99 -- "$(dirname -- "$1")/$(basename -- "$1" .c).c" && mv a.out "$(basename -- "$1" .c)"
The EXAMPLES section of the basename() function (see the System Interfaces volume of POSIX.1-2017, basename()) includes a table showing examples of the results of processing several sample pathnames by the basename() and dirname() functions and by the basename and dirname utilities.
The behaviors of basename and dirname have been coordinated so that when string is a valid pathname:
$(basename -- "string")
would be a valid filename for the file in the directory:
$(dirname -- "string")
This would not work for the early proposal versions of these utilities due to the way it specified handling of trailing <slash> characters.
Since the definition of pathname specifies implementation-defined behavior for pathnames starting with two <slash> characters, this volume of POSIX.1-2017 specifies similar implementation-defined behavior for the basename and dirname utilities.
Section 2.5, Parameters and Variables, dirname
The Base Definitions volume of POSIX.1-2017, Section 3.271, Pathname, Chapter 8, Environment Variables
The System Interfaces volume of POSIX.1-2017, basename(), dirname()
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. 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.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .
basename(3p), dirname(1p), dirname(3p).