bash 模式/启用模式

INVOCATION
       A login shell is one whose first character of argument zero is a -, or one started with the --login option.

       An interactive shell is one started without non-option arguments and without the -c option whose standard  input  and  error      are both connected to terminals (as determined by isatty(3)), or one started with the -i option.  PS1 is set and $- includes  i if bash is interactive, allowing a shell script or a startup file to test this state.

       The following paragraphs describe how bash executes its startup files.  If any of the files exist but cannot be  read,  bash  reports an error.  Tildes are expanded in file names as described below under Tilde Expansion in the EXPANSION section.

       When  bash  is  invoked as an interactive login shell, or as a non-interactive shell with the --login option, it first reads      and executes commands from the file /etc/profile, if that file exists.  After reading that file, it looks  for  ~/.bash_pro‐   file,  ~/.bash_login,  and  ~/.profile, in that order, and reads and executes commands from the first one that exists and is   readable.  The --noprofile option may be used when the shell is started to inhibit this behavior.

       When a login shell exits, bash reads and executes commands from the file ~/.bash_logout, if it exists.

       When an interactive shell that is not a login shell is started, bash reads and executes commands from  /etc/bash.bashrc  and     ~/.bashrc,  if  these  files  exist.  This may be inhibited by using the --norc option.  The --rcfile file option will force    bash to read and execute commands from file instead of /etc/bash.bashrc and ~/.bashrc.

       When bash is started non-interactively, to run a shell script, for example, it looks for the variable BASH_ENV in the  envi‐ronment, expands its value if it appears there, and uses the expanded value as the name of a file to read and execute.  Bash behaves as if the following command were executed:     if [ -n "$BASH_ENV" ]; 

then . "$BASH_ENV"; fi   but the value of the PATH variable is not used to search for the file name.

       If bash is invoked with the name sh, it tries to mimic the startup behavior of historical versions of sh as closely as  possible,  while  conforming  to  the POSIX standard as well.  When invoked as an interactive login shell, or a non-interactive   shell with the --login option, it first attempts to read and execute commands from  /etc/profile  and 
~/.profile,  in  that order.  The --noprofile option may be used to inhibit this behavior.  When invoked as an interactive shell with the name sh,  bash looks for the variable ENV, expands its value if it is defined, and uses the expanded value as the name of  a  file  to read  and  execute.  Since a shell invoked as sh does not attempt to read and execute commands from any other startup files,   the --rcfile option has no effect.  A non-interactive shell invoked with the name sh does not  attempt  to  read  any  other      startup files.  When invoked as sh, bash enters posix mode after the startup files are read.

       When  bash  is  started  in  posix  mode, as with the --posix command line option, it follows the POSIX standard for startup  files.  In this mode, interactive shells expand the ENV variable and commands are read and executed from the file whose name   is the expanded value.  No other startup files are read.

       Bash  attempts to determine when it is being run with its standard input connected to a network connection, as when executed  by the remote shell daemon, usually rshd, or the secure shell daemon sshd.  If bash determines it is being run in this fashion,  it  reads  and  executes commands from ~/.bashrc and ~/.bashrc, if these files exist and are readable. 

 It will not do   this if invoked as sh.  The --norc option may be used to inhibit this behavior, and the --rcfile option may be used to force   another file to be read, but rshd does not generally invoke the shell with those options or allow them to be specified.

       If  the  shell is started with the effective user (group) id not equal to the real user (group) id, and the -p option is not     supplied, no startup files are read, shell functions are not  inherited  from  the  environment,  the  SHELLOPTS,  BASHOPTS,     CDPATH,  and  GLOBIGNORE  variables, if they appear in the environment, are ignored, and the effective user id is set to the   real user id.  If the -p option is supplied at invocation, the startup behavior is the same, but the effective  user  id  is   not reset.