Return to Answer

[[ ! -t 0 ]] = Does standard input contains anything?

Your premise is wrong. This tests whether stdin is connected to a terminal/tty, not whether stdin contains anything. Because of the negation the command returns 0 (true) if stdin is not connected to a terminal.

See man test:

-t FD file descriptor FD is opened on a terminal

and man bash:

-t fd True if file descriptor fd is open and refers to a terminal.

With this in mind, re-evaluate your examples and you'll see they do indeed return the expected exit status.

The correct way to check if there's anything pending on stdin would be to use select(2), but that's not directly available in a shell such as bash. The closest I can suggest is to use read with an immediate timeout:

sleep 2
if read -t 0 _
then
    echo "Ready to receive data"
    IFS= read -r data
    echo "Received: $data"
fi

Run this, and then during the initial sleep, (a) try preloading stdin with some text, (b) do nothing.

Note that this will still hang (on the read -t 0 _) if you're reading from a pipe and there is no writer present. It will work as I've described if there is a writer present, even if that writer has not written anything to read.

Finally, responding to a comment on the original question. If you have ifne you can extend it trivially to let you use if...:

if ifne true; then ...; else ...; fi

[[ ! -t 0 ]] = Does standard input contains anything?

Your premise is wrong. This tests whether stdin is connected to a terminal/tty, not whether stdin contains anything. Because of the negation the command returns 0 (true) if stdin is not connected to a terminal.

See man test:

-t FD file descriptor FD is opened on a terminal

and man bash:

-t fd True if file descriptor fd is open and refers to a terminal.

With this in mind, re-evaluate your examples and you'll see they do indeed return the expected exit status.

The correct way to check if there's anything pending on stdin would be to use select(2), but that's not directly available in a shell such as bash. The closest I can suggest is to use read with an immediate timeout:

sleep 2
if read -t 0 _
then
    echo "Ready to receive data"
    IFS= read -r data
    echo "Received: $data"
fi

Run this, and then during the initial sleep, (a) try preloading stdin with some text, (b) do nothing.

Note that this will still hang (on the read -t 0 _) if you're reading from a pipe and there is no writer present. It will work as I've described if there is a writer present, even if that writer has not written anything to read.

[[ ! -t 0 ]] = Does standard input contains anything?

Your premise is wrong. This tests whether stdin is connected to a terminal/tty, not whether stdin contains anything. Because of the negation the command returns 0 (true) if stdin is not connected to a terminal.

See man test:

-t FD file descriptor FD is opened on a terminal

and man bash:

-t fd True if file descriptor fd is open and refers to a terminal.

With this in mind, re-evaluate your examples and you'll see they do indeed return the expected exit status.

The correct way to check if there's anything pending on stdin would be to use select(2), but that's not directly available in a shell such as bash. The closest I can suggest is to use read with an immediate timeout:

sleep 2
if read -t 0 _
then
    echo "Ready to receive data"
    IFS= read -r data
    echo "Received: $data"
fi

Run this, and then during the initial sleep, (a) try preloading stdin with some text, (b) do nothing.

Note that this will still hang (on the read -t 0 _) if you're reading from a pipe and there is no writer present. It will work as I've described if there is a writer present, even if that writer has not written anything to read.

[[ ! -t 0 ]] = Does standard input contains anything?

Your premise is wrong. This tests whether stdin is connected to a terminal/tty, not whether stdin contains anything. Because of the negation the command returns 0 (true) if stdin is not connected to a terminal.

See man test:

-t FD file descriptor FD is opened on a terminal

and man bash:

-t fd True if file descriptor fd is open and refers to a terminal.

With this in mind, re-evaluate your examples and you'll see they do indeed return the expected exit status.

The correct way to check if there's anything pending on stdin would be to use select(2), but that's not directly available in a shell such as bash. The closest I can suggest is to use read with an immediate timeout:

sleep 2
if read -t 0 _
then
    echo "Ready to receive data"
    IFS= read -r data
    echo "Received: $data"
fi

Run this, and then during the initial sleep, (a) try preloading stdin with some text, (b) do nothing.

Note that this will still hang (on the read -t 0 _) if you're reading from a pipe and there is no writer present. It will work as I've described if there is a writer present, even if that writer has not written anything to read.

Finally, responding to a comment on the original question. If you have ifne you can extend it trivially to let you use if...:

if ifne true; then ...; else ...; fi

[[ ! -t 0 ]] = Does standard input contains anything?

Your premise is wrong. This tests whether stdin is connected to a terminal/tty, not whether stdin contains anything. Because of the negation the command returns 0 (true) if stdin is not connected to a terminal.

See man test:

-t FD file descriptor FD is opened on a terminal

and man bash:

-t fd True if file descriptor fd is open and refers to a terminal.

With this in mind, re-evaluate your examples and you'll see they do indeed return the expected exit status.

The correct way to check if there's anything pending on stdin would be to use select(2), but that's not directly available in a shell such as bash. The closest I can suggest is to use read with an immediate timeout:

sleep 2
if read -t 0 _
then
    echo "Ready to receive data"
    IFS= read -r data
    echo "Received: $data"
fi

Run this, and then during the initial sleep, (a) try preloading stdin with some text, (b) do nothing.

[[ ! -t 0 ]] = Does standard input contains anything?

Your premise is wrong. This tests whether stdin is connected to a terminal/tty, not whether stdin contains anything. Because of the negation the command returns 0 (true) if stdin is not connected to a terminal.

See man test:

-t FD file descriptor FD is opened on a terminal

and man bash:

-t fd True if file descriptor fd is open and refers to a terminal.

With this in mind, re-evaluate your examples and you'll see they do indeed return the expected exit status.

The correct way to check if there's anything pending on stdin would be to use select(2), but that's not directly available in a shell such as bash. The closest I can suggest is to use read with an immediate timeout:

sleep 2
if read -t 0 _
then
    echo "Ready to receive data"
    IFS= read -r data
    echo "Received: $data"
fi

Run this, and then during the initial sleep, (a) try preloading stdin with some text, (b) do nothing.

Note that this will still hang (on the read -t 0 _) if you're reading from a pipe and there is no writer present. It will work as I've described if there is a writer present, even if that writer has not written anything to read.