空命令：的使用

root@james-desktop:~# help :
:: :
    Null command.
    No effect; the command does nothing.
    Exit Status:
    Always succeeds.

~ # : sdafsadf
~ #

~ # echo sdfsadf
sdfsadf

控制调试信息输出：

#!/bin/sh

if [ -z "$SILENT" ]; then
    ECHO=echo
else
    ECHO=:
fi

$ECHO hello

root@james-desktop:~# cat /etc/init.d/smbd
#!/bin/sh -e
# upstart-job
#
# Symlink target for initscripts that have been converted to Upstart.

set -e

INITSCRIPT="$(basename "$0")"
JOB="${INITSCRIPT%.sh}"

if [ "$JOB" = "upstart-job" ]; then
    if [ -z "$1" ]; then
        echo "Usage: upstart-job JOB COMMAND" 1>&2
        exit 1
    fi

    JOB="$1"
    INITSCRIPT="$1"
    shift
else
    if [ -z "$1" ]; then
        echo "Usage: $0 COMMAND" 1>&2
        exit 1
    fi
fi

COMMAND="$1"
shift

if [ -z "$DPKG_MAINTSCRIPT_PACKAGE" ]; then
        ECHO=echo
else
        ECHO=:
fi

$ECHO "Rather than invoking init scripts through /etc/init.d, use the service(8)"
$ECHO "utility, e.g. service $INITSCRIPT $COMMAND"

case $COMMAND in
status)
    $ECHO
    $ECHO "Since the script you are attempting to invoke has been converted to an"
    $ECHO "Upstart job, you may also use the $COMMAND(8) utility, e.g. $COMMAND $JOB"
    $COMMAND "$JOB"
    ;;
start|stop|restart)
    $ECHO
    $ECHO "Since the script you are attempting to invoke has been converted to an"
    $ECHO "Upstart job, you may also use the $COMMAND(8) utility, e.g. $COMMAND $JOB"
    PID=$(status "$JOB" 2>/dev/null | awk '/[0-9]$/ { print $NF }')
    if [ -z "$PID" ] && [ "$COMMAND" = "stop" ]; then
        exit 0
    elif [ -n "$PID" ] && [ "$COMMAND" = "start" ]; then
        exit 0
    elif [ -z "$PID" ] && [ "$COMMAND" = "restart" ]; then
        start "$JOB"
        exit 0
    fi
    $COMMAND "$JOB"
    ;;
reload|force-reload)
    $ECHO
    $ECHO "Since the script you are attempting to invoke has been converted to an"
    $ECHO "Upstart job, you may also use the $COMMAND(8) utility, e.g. $COMMAND $JOB"
    reload "$JOB"
    ;;
*)
    $ECHO
    $ECHO "The script you are attempting to invoke has been converted to an Upstart" 1>&2
    $ECHO "job, but $COMMAND is not supported for Upstart jobs." 1>&2
    exit 1
esac