APLawrence.com -  Resources for Unix and Linux Systems, Bloggers and the self-employed

2003/11/04 exec

© November 2003 Tony Lawrence

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.

Got something to add? Send me email.

(OLDER)    <- More Stuff -> (NEWER)    (NEWEST)   

Printer Friendly Version

-> exec

Inexpensive and informative Apple related e-books:

Take Control of the Mac Command Line with Terminal, Second Edition

Take Control of Parallels Desktop 12

Take Control of Preview

Photos for Mac: A Take Control Crash Course

Take Control of Upgrading to El Capitan

More Articles by © 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?


Printer Friendly Version

Have you tried Searching this site?

This is a Unix/Linux resource website. It contains technical articles about Unix, Linux and general computing related subjects, opinion, news, help files, how-to's, tutorials and more.

Contact us

Printer Friendly Version

Lawyer — One who protects us against robbers by taking away the temptation. (H.L. Mencken)

Linux posts

Troubleshooting posts

This post tagged:




Unix/Linux Consultants

Skills Tests

Unix/Linux Book Reviews

My Unix/Linux Troubleshooting Book

This site runs on Linode