bash: general

• to login
  • Interactive shell
    • ~/.bashrc: launch settings, will execuate these command while launch
  • login shell
    • ~/.bash_profile: login settings, will execuate these command while login
  • GNOME, KDE..
    • when login these Graphic Interface, mostly, the created terminal is not login shell
    • So they will not read .profile or .bash_profile
  • ssh remote login system will read .profile 

• knowledge
  • file hander is the integer which is related with input stream and output stream

• Terminal prompt
  • $: for normal user
  • #: for root user

• pay attention
  • var = value: with a blank space on both side of "=": to test equivalence
  • var=value: no blank space: assignment
  • [$var -eq 0 ] or [ $var -eq 0]: must have a space beside [ or ]

• system issues
  • /bin: binary files, using commands here, shell will always launch a new process. So using commands here is slower to use built-in commands

• special path
  • /proc/$PID/environ: process environment
  • /etc/environment
  • /etc/security/limits.conf
  • /dev/null: discard all received data
  • /dev/stdin, /dev/stdout, /dev/stderr

• exit status
  • command fail: return none zero value
  • command pass: return 0
  • $?: to get the exit status

• echo "<string>" or '<string>' or <string>
  • " " will explain special charcter of <string>
  • ' ' will not explain anything
  • <string> can't contain semicolon

• special character
  • $?: to get exit status
  • $0: the script's name
  • $n: nth parameter
  • $@: parameters, to recognize as a list as "$1" "$2" "$3"...
  • $*: parameters, to recognize as "$1c$2c$3", c is the first character in IFS
  • $#: number of parameter
  • $!: get the latest background run process's PID
  • : : tell shell to do nothing
  • IFS: Internal Field Separator:defalult is a blank character (\n, \t, \s)
  • PS1: environment to set shell prompt in command line 

• environment variable
  • HOME
  • PWD
  • USER
  • UID: user id
  • SHELL
  • PATH: exectuable files' path
  • LD_LIBRARY_PATH: lib files' path