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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(r) 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(r) does **not** specify any options for ''exec'' (like ''-c'', ''-l'', ''-a NAME'').

See also

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