exec - execute commands and manipulate file descriptors
exec [command [argument ...] ]
This command is a built-in command of the shell.
The exec(1) command runs the specified command with the specified argument. The command replaces the current shell without creating a new process.
If no command is specified, exec(1) manipulates file descriptors in the shell. (It is recommended that you read the section Redirections in the manual page topic sh(1) page before trying complicated tasks with exec(1).)
If a command was specified, the return value is the return value of the process which replaced the shell, or 127 if the command was not found.
If no command was specified, exec(1) returns 0 for success. If a redirection error occurred, the shell itself exits with an error in the range of 1-125. See the shell reference page for more information.
Use the following to replace the current shell with a copy of the Korn shell:
exec ksh
The exec(1) is frequently used in shell scripts to manipulate file descriptors. To open a file for reading on file descriptor 4, type the following:
exec 4< inputfile
To open the file outputfile for writing on file descriptor 3, type:
exec 3> outputfile
To close the file associated with file descriptor 9, type:
exec 9<&-
More complicated examples use the m>&n syntax, which points file descriptor n to the same file as descriptor m For example, to swap standard output and standard error for a process, type:
command 3>&2 2>&1 1>&3
The first redirection assigns standard error to descriptor 3, the
second assigns standard input to descriptor 2, and the third
assigns descriptor 3 (the original standard error) to descriptor 1.
File descriptor 3 is necessary as a placeholder; otherwise (if you
had done 2>&1 1>&2) standard output
would have been assigned to 2 along with standard error, and
standard error and standard output would have been assigned
to file descriptor 1.
The following shell script excerpt reads a line from the file $script, takes output from the terminal, and then combines them in the file $merge. Note that the redirection for the loop is done at the end of the loop:
exec 4<&0 # save original stdin as descriptor 4
while read prompt
do
echo -n "$prompt"
exec 5<&0 # save $script file descriptor in 5
exec 0<&4 # use the original input to read $reply
read reply
exec 0<&5 # reconnect $script file to stdin
echo "$prompt $reply" 1>&3 ;;
done < "$script" 3> "$merge"
A better example of this script can be found in the book UNIX Power Tools published by O'Reilly & Associates/Bantam.
ksh(1)
sh(1)