How to correctly add a path to PATH?

The simple stuff

PATH=$PATH:~/opt/bin

or

PATH=~/opt/bin:$PATH

depending on whether you want to add ~/opt/bin at the end (to be searched after all other directories, in case there is a program by the same name in multiple directories) or at the beginning (to be searched before all other directories).

You can add multiple entries at the same time. PATH=$PATH:~/opt/bin:~/opt/node/bin or variations on the ordering work just fine. Don't put export at the beginning of the line as it has additional complications (see below under “Notes on shells other than bash”).

If your PATH gets built by many different components, you might end up with duplicate entries. See How to add home directory path to be discovered by Unix which command? and Remove duplicate $PATH entries with awk command to avoid adding duplicates or remove them.

Some distributions automatically put ~/bin in your PATH if it exists, by the way.

Where to put it

Put the line to modify PATH in ~/.profile, or in ~/.bash_profile if that's what you have.

Note that ~/.bash_rc is not read by any program, and ~/.bashrc is the configuration file of interactive instances of bash. You should not define environment variables in ~/.bashrc. The right place to define environment variables such as PATH is ~/.profile (or ~/.bash_profile if you don't care about shells other than bash). See What's the difference between them and which one should I use?

Don't put it in /etc/environment or ~/.pam_environment: these are not shell files, you can't use substitutions like $PATH in there. In these files, you can only override a variable, not add to it.

Potential complications in some system scripts

You don't need export if the variable is already in the environment: any change of the value of the variable is reflected in the environment.¹ PATH is pretty much always in the environment; all unix systems set it very early on (usually in the very first process, in fact).

At login time, you can rely on PATH being already in the environment, and already containing some system directories. If you're writing a script that may be executed early while setting up some kind of virtual environment, you may need to ensure that PATH is non-empty and exported: if PATH is still unset, then something like PATH=$PATH:/some/directory would set PATH to :/some/directory, and the empty component at the beginning means the current directory (like .:/some/directory).

if [ -z "${PATH-}" ]; then export PATH=/usr/local/bin:/usr/bin:/bin; fi

Notes on shells other than bash

In bash, ksh and zsh, export is special syntax, and both PATH=~/opt/bin:$PATH and export PATH=~/opt/bin:$PATH do the right thing even. In other Bourne/POSIX-style shells such as dash (which is /bin/sh on many systems), export is parsed as an ordinary command, which implies two differences:

• ~ is only parsed at the beginning of a word, except in assignments (see How to add home directory path to be discovered by Unix which command? for details);
• $PATH outside double quotes breaks if PATH contains whitespace or \[*?.

So in shells like dash, export PATH=~/opt/bin:$PATH sets PATH to the literal string ~/opt/bin/: followed by the value of PATH up to the first space. PATH=~/opt/bin:$PATH (a bare assignment) doesn't require quotes and does the right thing. If you want to use export in a portable script, you need to write export PATH="$HOME/opt/bin:$PATH", or PATH=~/opt/bin:$PATH; export PATH (or PATH=$HOME/opt/bin:$PATH; export PATH for portability to even the Bourne shell that didn't accept export var=value and didn't do tilde expansion).

¹ _{This wasn't true in Bourne shells (as in the actual Bourne shell, not modern POSIX-style shells), but you're highly unlikely to encounter such old shells these days.}

Either way works, but they don't do the same thing: the elements of PATHare checked left to right. In your first example, executables in ~/opt/bin will have precedence over those installed, for example, in /usr/bin, which may or may not be what you want.

In particular, from a safety point of view, it is dangerous to add paths to the front, because if someone can gain write access to your ~/opt/bin, they can put, for example, a different ls in there, which you'd then probably use instead of /bin/ls without noticing. Now imagine the same for ssh or your browser or choice... (The same goes triply for putting . in your path.)

The bullet-proof way of Appending/Prepending

Try not using

PATH=$PATH:~/opt/bin

or

PATH=~/opt/bin:$PATH

Why? There are a lot of considerations involved in the choice of appending versus prepending. Many of them are covered in other answers, so I will not repeat them here.

An important point is that, even if system scripts do not use this (I wonder why)^*1, the bullet-proof way to add a path (e.g., ~/opt/bin) to the PATH environment variable is

PATH="${PATH:+${PATH}:}~/opt/bin"

for appending (instead of PATH="$PATH:~/opt/bin") and

PATH="~/opt/bin${PATH:+:${PATH}}"

for prepending (instead of PATH="~/opt/bin:$PATH")

This avoids the spurious leading/trailing colon when $PATH is initially empty, which can have undesired side effects and can become a nightmare, elusive to find (this answer briefly deals with the case the awk-way).

Explanation (from Shell Parameter Expansion):

${parameter:+word}

If parameter is null or unset, nothing is substituted, otherwise the expansion of word is substituted.

Thus, ${PATH:+${PATH}:} is expanded to:

1. nothing, if PATH is null or unset,
2. ${PATH}:, if PATH is set.

Note: This is for bash.

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^*1 I have just found that scripts like `devtoolset-6/enable` actually use this,

$ cat /opt/rh/devtoolset-6/enable
# General environment variables
export PATH=/opt/rh/devtoolset-6/root/usr/bin${PATH:+:${PATH}}
...