Shell Programming and Scripting

In end of https://www.unix.com/shell-programmin...les-shell.html

mjd_tech gives script which can read some values directly without manually input, but if no value is the right one, my understand is, it will on hold for waiting the next input, but when I run below codes, it keeps running and refresh my screen.

Code:

while [ "$line" != "Fred" ]; do
    read line
    echo $line
done << EOF
Wilma
Barney
#Fred
Betty
EOF

Why does it behave like that?

It won't. You feed the while loop with the content of here-document, so it processes it line-by-line.
Read builtin takes input from a file descriptor. A file descriptor is either an open file, and it takes a line from there, or stdin, and it waits for input from terminal. In your case it's a file (you are creating a here-document with << operator), that's where read is reading from, and it wouldn't make sense for it to wait for stdin.

Thanks, so after read command get EOF , what will happen then?

Will EOF tell the read command, the here-document is finished, no read from now on, why read still keep reading with "null" value.

Hi


It behaves like that becuause there is no condition which will make the read statement to stop reading. Even after the here document is complete, read tries to read from the stdin, does not find any, goes over to the while loop condition which is true, reads again, and this cycle goes on.

For your script to go ahead, you have to make the while loop break when there is no more input:

Code:

while [ "$line" != "Fred" ]; do
    read line
    [ -z "$line" ] && break
    echo $line
done <<EOF
Wilma
Barney
Frea
Betty
EOF

Guru

So if follow your explain, why read is not on hold to wait the input?

Hi

When 'read' reads from standard input, it holds, does not hold when it is reading from a file.

Try this, you will understand:

Code:

$ cat file
Wilma
Barney

Code:

$ while [ 1 ]
> do
>  read line
>  echo $line
> done < file

Guru.

As others mentioned, read reads lines from standard input. In this case, standard input is coming from the here-document. Hence, read will always read from the here-doc. read will wait for the input (until a time either specified by TMOUT or that overridden by -t option) provided the source (in this case, the here-doc) has not been "exhausted". In your case, after printing out Betty, the here-doc has been "exhausted" and read tries to read from this source and reads a null value with an exit status of 1 and since you have not specified any exit condition for the loop for this case, the loop just prints out the null value of line.