help bg

bg: bg [job_spec ...]
    Move jobs to the background.

    Place the jobs identified by each JOB_SPEC in the background, as if they
    had been started with `&'.  If JOB_SPEC is not present, the shell's notion
    of the current job is used.

    Exit Status:
    Returns success unless job control is not enabled or an error occurs.

help fg

fg: fg [job_spec]
    Move job to the foreground.

    Place the job identified by JOB_SPEC in the foreground, making it the
    current job.  If JOB_SPEC is not present, the shell's notion of the
    current job is used.

    Exit Status:
    Status of command placed in foreground, or failure if an error occurs.

help jobs

jobs: jobs [-lnprs] [jobspec ...] or jobs -x command [args]
    Display status of jobs.

    Lists the active jobs.  JOBSPEC restricts output to that job.
    Without options, the status of all active jobs is displayed.

    Options:
      -l        lists process IDs in addition to the normal information
      -n        lists only processes that have changed status since the last
        notification
      -p        lists process IDs only
      -r        restrict output to running jobs
      -s        restrict output to stopped jobs

    If -x is supplied, COMMAND is run after all job specifications that
    appear in ARGS have been replaced with the process ID of that job's
    process group leader.

    Exit Status:
    Returns success unless an invalid option is given or an error occurs.
    If -x is used, returns the exit status of COMMAND.

fg、bg、jobs、&、ctrl + z都是跟系统任务有关的，虽然现在基本上不怎么需要用到这些命令，但学会了也是很实用的

1. & 最经常被用到

这个用在一个命令的最后，可以把这个命令放到后台执行

2. ctrl + z

可以将一个正在前台执行的命令放到后台，并且暂停

3. jobs

查看当前有多少在后台运行的命令

4. fg

将后台中的命令调至前台继续运行 如果后台中有多个命令，可以用 fg %jobnumber将选中的命令调出，%jobnumber是通过jobs命令查到的后台正在执行的命令的序号(不是pid)

5. bg

将一个在后台暂停的命令，变成继续执行 如果后台中有多个命令，可以用bg %jobnumber将选中的命令调出，%jobnumber是通过jobs命令查到的后台正在执行的命令的序号(不是pid)