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2003/11/04 exec

In Bourne-like shells (sh, bash, ksh), exec serves two functions:

It can replace your current program with another, or can help you control input and output more easily.

The first use is often used at the last line of a .profile:

exec someprog

Without the "exec", someprog would still run, but when the user quit, they'd be returned to .profile which, having nothing else to do, would drop them to a shell prompt. With the "exec", leaving the program brings them to login.

For redirecting input, exec is very handy. The following little script happily echoes whatever you type, unless you type Q:

date > tt
exec 5<&0
while true
read stuff
echo $stuff
exec <&5
case $stuff in 
 [qQ]) exec < tt;;

This illustrates how you can move back and forth between different input sources. The "exec 5<&0" saves the current standard input so that we can later restore it (exec <&5). If the input sees Q, then the file tt is read instead.

If you do

exec > tt

the output of "ls" will be in tt (do "exec > /dev/tty" to return things to normal).

You can specifically close a file descriptor too:

exec <&-

closes STDIN.

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© Tony Lawrence

It's not strictly true to say that "exec > /dev/tty" returns things to normal. I did a quick test and found that once you do this, you can't then redirect the output from the script to a file at the command line level...any output will pop up on the screen.

Anyone know a better way of doing it?


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