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.TH "ISATTY" "1" "Nov 13, 2025" "4.2" "fish-shell"
.SH NAME
isatty \- test if a file descriptor is a terminal
.SH SYNOPSIS
.nf
\fBisatty\fP [\fIFILE_DESCRIPTOR\fP]
.fi
.sp
.SH DESCRIPTION
.sp
\fBisatty\fP tests if a file descriptor is a terminal (as opposed to a file). The name is derived from the system call of the same name, which for historical reasons refers to a teletypewriter (TTY).
.sp
\fBFILE DESCRIPTOR\fP may be either the number of a file descriptor, or one of the strings \fBstdin\fP, \fBstdout\fP, or \fBstderr\fP\&. If not specified, zero is assumed.
.sp
If the specified file descriptor is a terminal device, the exit status of the command is zero. Otherwise, the exit status is non\-zero. No messages are printed to standard error.
.sp
The \fB\-h\fP or \fB\-\-help\fP option displays help about using this command.
.SH EXAMPLES
.sp
From an interactive shell, the commands below exit with a return value of zero:
.INDENT 0.0
.INDENT 3.5
.sp
.EX
isatty
isatty stdout
isatty 2
echo | isatty 1
.EE
.UNINDENT
.UNINDENT
.sp
And these will exit non\-zero:
.INDENT 0.0
.INDENT 3.5
.sp
.EX
echo | isatty
isatty 9
isatty stdout > file
isatty 2 2> file
.EE
.UNINDENT
.UNINDENT
.SH COPYRIGHT
fish-shell developers
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