Command line delimiters

Number of ways to indicate the end of an individual command.

<cr> - carriage return. 
Command runs and prompt returns on new line ready for next prompt.

; - semi-colon.
Can be used to separate several commands on a single line. 

Each command will complete or fail before next command runs.

& - ampersand
Each command will launch in background.

Next command in sequence will not wait for previous command to complete.

However, if background command needs standard input, it will stop.

& can be used after a single command.

| - pipe
Pipe connects standard output of command to standard input of second command.

|& - pipe with errors.
Combines both standard out and error when piping.

To redirect only errors, use & to tweak file descriptor assignments.

cmd 2>&1 >/dev/null | cmd2
# This reassigns std-err file descriptor to std-out.
# The redirect real std-out to null (you can use another filename).


Conditional delimiters

When a command runs, it sets a success status.
  Either 
    0 for success

  Or 
    value between 1-255 for failure. 

  Command decides value.

To see the status of last command run, run :

  echo $?

It will print a value.

The status value can be used to determine if next command runs.

&& - AND
Used between commands, the second command will run only if 1st completes
  successfully.

|| - OR
Used between commands, the second command will run only if 1st completes
  fails.

Daisy chaining these works but not always in an obvious way.

# list who is logged on,  who
# eliminate all IDs beginning with z  grep "^[^z]" and put in file logons
##   "^[^z]" -- match any lines beginning a character that is not z
# If grep succeeded in finding at least one non-z entry, 
#   sort the logons file 
# Else
#   # something went wrong, either grep or sort failed 
#   remove the logons file (it's empty)

who | grep "^[^z]" > logons && sort -o logons logons || rm logons


( ) - parentheses or group.
Parentheses generates a sub-shell or 2nd copy of bash.
But unlike just starting a second copy of bash from the command line,
  local variables are passed in.
However, 
  variables created inside sub-shell perish on termination of sub-shell.
  changes in a variable's value are lost on termination of sub-shell.

export has no effect on this.

var=7; ( ps; echo $var; var=9; echo $var ); echo $var
  PID TTY          TIME CMD
 1180 pts/0    00:00:00 bash
 5550 pts/0    00:00:00 bash
 5551 pts/0    00:00:00 ps
7
9
7

Note that a second bash is started.


A terminating sub-shell generates a return code which can be tested outside
of the parentheses. 

This will be the status of the last command run.

The exit command can be used terminate a sub-shell 
  and set a desired return code.

./f1 || ( echo "oops"; exit 1 ) && echo "good" || echo "bad"
# If the echo displays correctly, the current cmd status is success.
# The 'exit 1' sets the status of the terminating sub-shell  fail.
# The second OR tests the success of the sub-shell..

Changes in current working directory are also lost on termination of sub-shell.

Try the following, there is no line break in this  : 

var=bob; (cd class; pwd; var="in"; (cd 330; pwd; ps; echo $var; var="really"; echo $var ); echo $var;  pwd ); pwd; echo $var