Exploring $HOME/.profile

: ${HOME='/root'} # no error message

• If HOME is not set, assign ‘/root’ to HOME and $HOME will be substituted with /root
• The colon lets you set HOME without printing an error message. The command interpreter tries to execute $HOME, which is substituted as /root, which by itself is not a command.

${TEST='ls'}
$TEST

• TEST isn’t set, so it is assigned ‘ls’ and substituted. The command interpreter recognizes ‘ls’ as a command and lists the directory. Same deal with $TEST by itself.

echo ${HOME='/bin'}

• This returns /root
• If HOME is already set, $HOME is substituted with ‘/root’.
• If HOME is not set, then HOME is assigned ‘/bin’ and substituted.

umask 022

• The umask command sets the bits to be turned off on file creation.
• 022 turns off write permissions for group and others.
• As umask(2) states, “write access for the owner only.”