How shell works

1
How shell works

• Shell reads in input, parse process it, the
  resulting string is taken to be argv and
  executed
• echo a ? shell sees echo aecho gets
  argvecho a, prints out a
• ab echo a ? shell sees echo a, change it
  to echo becho gets argvecho b, prints
  out b

2
How shell works

• ab bc echo \a? shell sees echo
  \ashell interprets \ as the char shell
  interprets a as bthe resulting string is
  echo becho gets argvecho becho
  prints out b on the screen
• What if I want the value of b printed on the
  screen?

3
How eval works

• ab bc eval echo \a? shell sees eval echo
  \ashell takes away evalshell interprets \
  as the char shell interprets a as bthe
  resulting string is echo bbecause of eval,
  shell parses this string once againshell
  interprets b as cecho gets argvecho
  cecho prints out c on the screen

4
Command Path

• The shell variable path specifies where to
  search for commands
• If not found, shells prints Command not found
• To be very specific, use full pathname, such as
  /sbin/dumpon -v dumpdev

5
Notable Commands

• vinum
• swapon
• fsck
• mount ( fstab)
• find

6
Difference between . And Sh

• Both . script1 and sh script1 executes
  script1
• Both have the same permanent side effect, such as
  the removal of a file
• . does the work in the same shell
• sh does the work in a separate shell
• If what you want is the side effect within the
  shell, such as the value of a shell variable, use
  . instead of sh.