'\" et
.TH EVAL "1P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
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.
.\"
.SH NAME
eval
\(em construct command by concatenating arguments
.SH SYNOPSIS
.LP
.nf
eval \fB[\fIargument\fR...\fB]\fR
.fi
.SH DESCRIPTION
The
.IR eval
utility shall construct a command by concatenating
.IR argument s
together, separating each with a
<space>
character.
The constructed command shall be read and executed by the shell.
.SH OPTIONS
None.
.SH OPERANDS
See the DESCRIPTION.
.SH STDIN
Not used.
.SH "INPUT FILES"
None.
.SH "ENVIRONMENT VARIABLES"
None.
.SH "ASYNCHRONOUS EVENTS"
Default.
.SH STDOUT
Not used.
.SH STDERR
The standard error shall be used only for diagnostic messages.
.SH "OUTPUT FILES"
None.
.SH "EXTENDED DESCRIPTION"
None.
.SH "EXIT STATUS"
If there are no
.IR argument s,
or only null
.IR argument s,
.IR eval
shall return a zero exit status; otherwise, it shall return the exit
status of the command defined by the string of concatenated
.IR argument s
separated by
<space>
characters, or a non-zero exit status if the concatenation could not
be parsed as a command and the shell is interactive (and therefore did
not abort).
.SH "CONSEQUENCES OF ERRORS"
Default.
.LP
.IR "The following sections are informative."
.SH "APPLICATION USAGE"
Since
.IR eval
is not required to recognize the
.BR \(dq--\(dq 
end of options delimiter, in cases where the argument(s) to
.IR eval
might begin with
.BR '-' 
it is recommended that the first argument is prefixed by a string that
will not alter the commands to be executed, such as a
<space>
character:
.sp
.RS 4
.nf

eval " $commands"
.fi
.P
.RE
.P
or:
.sp
.RS 4
.nf

eval " $(some_command)"
.fi
.P
.RE
.SH EXAMPLES
.LP
.nf
foo=10 x=foo
y=\(aq$\(aq$x
echo $y
\fB$foo\fR
eval y=\(aq$\(aq$x
echo $y
\fB10\fR
.fi
.SH "RATIONALE"
This standard allows, but does not require,
.IR eval
to recognize
.BR \(dq--\(dq .
Although this means applications cannot use
.BR \(dq--\(dq 
to protect against options supported as an extension (or errors reported
for unsupported options), the nature of the
.IR eval
utility is such that other means can be used to provide this protection
(see APPLICATION USAGE above).
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "Section 2.14" ", " "Special Built-In Utilities"
.\"
.SH COPYRIGHT
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 .
.PP
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 .