bash.txt

|| vs if/else pitfall
---------------------

touch /tmp/brr && ech 'created' || echo 'fail'

Here the touch succedes so we only want ech 'created' to be executed
but since that is wrong we also get the second echo

compare with:
if touch /tmp/brr
then
  ech 'created'
else
  echo 'fail'
fi

Quotes
------

Q. In FAQ096, why is this ok? ssh localhost '~/bin/args make CFLAGS="-g -O"'
   Why do we end up with 2 args and not 3 because of remote word splitting?
A. it gets parsed by the local shell, then the remote shell
   it'll run                             "$SHELL" -c '~/bin/args make CFLAGS="-g -O"' on the remote end
   if you omit the '' quotes, it'll run  "$SHELL" -c '/home/localhomedir/bin/args make CFLAGS=-g -O'

nesting single quotes:
   ssh 'grep -iE '\''blah'\'' file'
1. ssh 'grep -iE '  'blah'  ' file' -> break it in 3 sets of quotes
2.                \''blah'\'        -> use literal 's that bash won't parse
   ssh grep -iE 'blah' file         -> our remote command

Completion
----------

complete -d -Fmyfunc cd: cd will use myfunc and dir completion
compgen -d m: generate completion list of dirs    named m... (<=> -Adirectory)
compgen -a m: generate completion list of aliases named m...
compgen must put the possible completions in the COMPREPLY array
COMP_WORDS: command line args during completion - [ cd m ]<tab>
COMP_CWORD: index in COMP_WORDS of the word containing the cursor

Misc
----

parent shell PID: $$, current subshell: BASHPID

shell:

[ x"$var" = xyes ] && ... # the x at the front prevents a leading dash from being picked up as an option to test (-n, -z)

# Use PROMPT_COMMAND to aggregate users' history into a single file
# /var/log/user-history.log                                  whoami | bash PID |         history 1        |  $?
#                                                            oge    | [21118]: | 2013-09-09_10:46:34 su - | [1]
export PROMPT_COMMAND='RETRN_VAL=$?; logger -p local6.debug "$LOGNAME [$$]: $(history 1 | command sed "s/^[ ]*[0-9]\+[ ]*//" ) [$RETRN_VAL]"; ...'

source aka .
tells Bash to read the commands in myscript and run them in the current shell
environment. Since the commands are run in the current shell, they can change
the current shell's variables, working directory, open file descriptors,
functions, etc.