The following 2 lines begin every Bash script in Plushu and its plugins:
#!/usr/bin/env bash
set -eo pipefail; [[ $PLUSHU_TRACE ]] && set -xThe following is the rationale for each part of this.
#!/usr/bin/env bash
We use /usr/bin/env to get the path to bash because certain systems may be set up so that a newer-installed version of bash is placed higher in $PATH than /usr/bin/bash. This is common in Mac OS X environments, which historically ship with an old (3.*) version of bash in /usr/bin, where developers often install a more recent one in their home directory (via brew or macports) to use when testing a script's correctness.
set -eo pipefail
We set -e so that any uncaught error in the script will cause it to exit, and we set -o pipefail so that an error in the middle of a pipeline will read as a failure as well (normally, the success or failure of a pipeline is decided by the last command in it).
Without exiting on uncaught errors, a script may continue execution after an unforeseen problem has arisen, further compounding whatever issue may have caused it to fail.
Using set -e as a standard practice is not universally agreed upon, but when used properly, it comes out better than the alternative.
[[ $PLUSHU_TRACE ]] && set -x
This line sets the script to output a trace of every command executed, for debugging. This allows a user to set "export PLUSHU_TRACE=true" in .plushurc to turn debug mode on for every action taken when running a command in Plushu.
(The plushu script itself places the second part of line 2 farther down in the script's execution, after the .plushurc that would set it has been sourced.)
Except for assignment, variables in Plushu are always quoted unless whitespace splitting or globbing is explicitly needed.
Even if a variable will never have a value with whitespace or glob characters, it is still quoted, for consistency. Also, you never know when some outside factor will break your assumptions about the "expected" value (see stuartpb/dokku-rethinkdb-plugin#4, where an unexpected "<no value>" resulted in a check against both "<no" and "value>").
Plushu only uses echo to output messages, never variables (see issue #22 for the history and reasoning for this).
Plushu uses Bash herestrings to pass a variable to a command via stdin (rather than piping a command), and printf '%s\n' to output a variable to stdout or a file.
All tests use the [[ builtin "extended test" construction. Comparison tests use "==" (as used for comparison in virtually every other language) rather than "=" (which is easily confused with the assignment operator).
Testing for empty or non-empty strings is done with -z and -n, respectively, rather than letting the string be tested without a flag (for essentially the same reason as why echo is not used for writing variables).
Using an && conditional as a short test is frowned upon (with the exception of the PLUSHU_TRACE test in the header), as it can result in side-effects like this when used as the last command of a script (which determines its exit code).
All command substitution should use $() rather than backticks. On top of being POSIX-standard and simpler to nest, $() constructs use the same backslash escaping rules as surrounding code. Backtick constructions have their own crazy rules about backslash escaping (see http://mywiki.wooledge.org/BashFAQ/082 and the manual).