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If job control is enabled (see the description of set -m), the fg utility shall move a background job from the current environment (see Section 2.12, Shell Execution Environment) into the foreground.
Using fg to place a job into the foreground shall remove its process ID from the list of those “known in the current shell execution environment”; see Section 22.214.171.124, Examples.
The following operand shall be supported:
Specify the job to be run as a foreground job. If no job_id operand is given, the job_id for the job that was most recently suspended, placed in the background, or run as a background job shall be used. The format of job_id is described in the Base Definitions volume of POSIX.1-2017, Section 3.204, Job Control Job ID.
The following environment variables shall affect the execution of fg:
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 fg utility shall write the command line of the job to standard output in the following format:
The standard error shall be used only for diagnostic messages.
The following exit values shall be returned:
An error occurred.
Consequences of Errors
If job control is disabled, the fg utility shall exit with an error and no job shall be placed in the foreground.
The following sections are informative.
The fg utility does not work as expected when it is operating in its own utility execution environment because that environment has no applicable jobs to manipulate. See the APPLICATION USAGE section for bg. For this reason, fg is generally implemented as a shell regular built-in.
The extensions to the shell specified in this volume of POSIX.1-2017 have mostly been based on features provided by the KornShell. The job control features provided by bg, fg, and jobs are also based on the KornShell. The standard developers examined the characteristics of the C shell versions of these utilities and found that differences exist. Despite widespread use of the C shell, the KornShell versions were selected for this volume of POSIX.1-2017 to maintain a degree of uniformity with the rest of the KornShell features selected (such as the very popular command line editing features).
Section 126.96.36.199, Examples, Section 2.12, Shell Execution Environment, bg, kill, jobs, wait
The Base Definitions volume of POSIX.1-2017, Section 3.204, Job Control Job ID, Chapter 8, Environment Variables
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 .
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