The exec builtin command

Synopsis

exec [-a NAME] [-cl] [COMMAND] [ARG...] [REDIRECTION...]

Description

The exec builtin command is used to

replace the shell with a given program (executing it, not as new process)
set redirections for the program to execute or for the current shell

If only redirections are given, the redirections affect the current shell without executing any program.

Options

Option	Description
`-a NAME`	Passes `NAME` as zeroth argument for the program to be executed
`-c`	Execute the program with an empty (cleared) environment
`-l`	Prepends a dash (`-`) to the zeroth argument of the program to be executed, similar to what the `login` program does

Exit status

on redirection errors it returns 1, otherwise 0
on exec failures:
- a non-interactive shell terminates; if the shell option execfail is set exec returns failure
- in an interactive shell, exec returns failure

Examples

Wrapper around a program

myprog=/bin/ls
echo "This is the wrapper script, it will exec $myprog"
 
# do some vodoo here, probably change the arguments etc.
# well, stuff a wrapper is there for
 
exec "$myprog" "$@"

Open a file as input for the script

# open it
exec 3< input.txt
 
# for example: read one line from the file(-descriptor)
read -u 3 LINE
# or
read LINE <&3
 
# finally, close it
exec 3<&-

Overall script logfile

To redirect the whole stdout and stderr of the shell or shellscript to a file, you can use the exec builtin command:

exec >/var/adm/my.log 2>&1
 
# script continues here...

Portability considerations

POSIX® specifies error code ranges:
- if exec can't find the program to execute, the error code shall be 126
- on a redirection error, the error code shall be between 1 and 125
the -a NAME option appeared in Bash 4.2-alpha
POSIX® does not specify any options for exec (like -c, -l, -a NAME).