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@fcrozat's original fix assumes `which` not to output anything to STDOUT
in case the command is not found. That is not necessarily true on all
systems. A better solution is to check the return value instead.

Fixes #4376

Fixes issue #4376

* Balk at incompatible Windows/MSYS git
* Test for chsh presence before trying to use it
* Replace non-portable `[[ ... ]]` and `[ x = *pattern* ]` constructs

Check if Zsh is installed before installing anything else.
New-commers, or people who don't read the disclaimers(ex. me) will go
straight to stack overflow if chsh -s returns an error.

Installation previously assumed the existence of a "chsh" command in the current
${PATH}. Since Cygwin does *NOT* provide this command, installation now tests
for the existence of this command before attempting to run it.

For safety, a umask of 022 prohibiting both group and other writability is now
enforced during OMZ installation. In theory, this should reduce the likelihood
of subsequent compinit() failures due to insecure directory permissions under
all platforms except for default Cygwin installations (in which Windows ACLs
override POSIX umasks).

Do not use terminal visual effects if not connected to a terminal.

fix test for chsh in install
in case $SHELL is zsh but not the last one in /etc/shells
fixes #3026 fixes #3779 fixes #3780

install.sh: Replace nonstandard "sed -i" with sed + mv commands. Makes portable to systems whose sed lacks "-i", like OpenBSD.

Use `--depth=1` to clone just the lastest version rather than every version in git.

Some environments (such as Android) does not have /usr/bin.

The intention of the redirection to /dev/null is to hide the output
'hash: no such command: git' since we rely on the exit status.

However, the output goes to stderr, so it's stderr that needs to be
redirected. For completeness, we redirect both stderr and stdout using
'2>&1'.

Example:

  [~]$ hash git > /dev/null
  [~]$ PATH=''
  [~]$ hash git > /dev/null
  hash: no such command: git
  [~]$ hash git > /dev/null 2>&1
  [~]$

Fixing a few quirks in the latest installer updates (quoting /Users/robbyrussell/.oh-my-zsh and such). Also mentioining our twitter account after install

Although the zshrc template adds a PATH= string, this is overwritten by the installer script.
This allows it to be placed anywhere in the file instead of having to append at the end.

Fix finding git issue. http://stackoverflow.com/questions/592620/check-if-a-program-exists-from-a-bash-script

Make tools/install.sh and tools/upgrade.sh pretty.

Updating install process to copy your current environments PATH and adding it to the bottom of ~/.zshrc.