Shell config subfiles

Large shell startup scripts (.bashrc, .profile) over about fifty lines or so with a lot of options, aliases, custom functions, and similar tweaks can get cumbersome to manage over time, and if you keep your dotfiles under version control it’s not terribly helpful to see large sets of commits just editing the one file when it could be more instructive if broken up into files by section.

Given that shell configuration is just shell code, we can apply the source builtin (or the . builtin for POSIX sh) to load several files at the end of a .bashrc, for example:

source ~/.bashrc.options
source ~/.bashrc.aliases
source ~/.bashrc.functions

This is a better approach, but it still binds us into using those filenames; we still have to edit the ~/.bashrc file if we want to rename them, or remove them, or add new ones.

Fortunately, UNIX-like systems have a common convention for this, the .d directory suffix, in which sections of configuration can be stored to be read by a main configuration file dynamically. In our case, we can create a new directory ~/.bashrc.d:

$ ls ~/.bashrc.d

With a slightly more advanced snippet at the end of ~/.bashrc, we can then load every file with the suffix .bash in this directory:

# Load any supplementary scripts
for config in "$HOME"/.bashrc.d/*.bash ; do
    source "$config"
unset -v config

Note that we unset the config variable after we’re done, otherwise it’ll be in the namespace of our shell where we don’t need it. You may also wish to check for the existence of the ~/.bashrc.d directory, check there’s at least one matching file inside it, or check that the file is readable before attempting to source it, depending on your preference.

The same method can be applied with .profile to load all scripts with the suffix .sh in ~/.profile.d, if we want to write in POSIX sh, with some slightly different syntax:

# Load any supplementary scripts
for config in "$HOME"/.profile.d/*.sh ; do
    . "$config"
unset -v config

Another advantage of this method is that if you have your dotfiles under version control, you can arrange to add extra snippets on a per-machine basis unversioned, without having to update your .bashrc file.

Here’s my implementation of the above method, for both .bashrc and .profile:

Safe MySQL updates

If you use the MySQL command line client a lot, you could consider adding the following to your .bashrc:

alias mysql='mysql --safe-updates'

Log out, then in again, or source your .bashrc file. This will quietly add in the --safe-updates parameter to any call you make for the mysql binary, which will prevent you from performing UPDATE or DELETE operations on any table if you neither specify a LIMIT condition nor a WHERE condition based on an indexed field. This stops you from accidentally deleting or updating every row in a table with queries like DELETE FROM `table` or UPDATE `table` SET `field` = 'value'.

If you really do want to delete a table’s entire contents, TRUNCATE `table` is more efficient anyway, and something you’re considerably less likely to fat-finger.

You can check this is loaded by typing alias with no arguments:

$ alias
alias mysql='mysql --safe-updates'

This very seldom gets in the way of actual operations you intend to run, and at the same time it prevents you from running potentially disastrous queries over your whole table. The MySQL manual lists it as for beginners, but regardless of your experience level in production environments this kind of safety valve can be a lifesaver, particularly if you’re not at your most lucid.

Like any Bash alias, if you want to call mysql directly without going through the alias, perhaps because you really do want to update a field in every single row of a table, then just prepend a backslash:

$ \mysql

Funnily enough, a valid alias of this option is --i-am-a-dummy. Pick whichever you think is more appropriate.

Bash command existence

If you set environment variables like your EDITOR in your .bashrc file that refer to commands that you expect to be available on the system, it’s prudent to check that an appropriate command actually exists before making the setting.

A common way of approaching this is using which, in a syntax similar to the below, which sets EDITOR to the first executable instance of vi found in PATH, or blank if not found:

EDITOR=$(which vi)

Because the behaviour of which can be unexpected, it’s better practice to use one of Bash’s builtins to do this, either command, type, or hash. I prefer using hash, which searches PATH for a command of the given name, and loads it into Bash’s command hash table if found. An implementation like the below works well:

if hash vi 2>/dev/null; then
    export EDITOR=vi

This ignores any error output from the hash call by redirecting it to the null device, and only defines the value for EDITOR if a matching command is found.

You can compact this syntax into one line:

hash vi 2>/dev/null && export EDITOR=vi

Thanks to commenter Yu-Jie Lin for pointing out the above abbreviation.

Automatic tmux titles

If you’re using tmux as a terminal multiplexer and keeping one window open per host, you might be manually renaming each window to feature the relevant hostname. This is a little annoying to do if you’re dealing with a lot of hosts, so it’s worthwhile to automate it.

In the tmux manual, the following escape code incantation is given to update the window title from within the terminal:

$ printf '\033kWINDOW_NAME\033\\'

In much the same way that you can update the title of an xterm-compatible terminal emulator with control codes as part of the $PS1 variable defining the prompt, you can update the title of a tmux window to the current hostname (or any other relevant text) automatically by prefixing this call to the $PROMPT_COMMAND. This is best done in your .bashrc. The below code assumes you are using either screen or screen-256color as your $TERM string in your .tmux.conf:

case "$TERM" in
        PROMPT_COMMAND="printf '\033k$(hostname)\033\\';"${PROMPT_COMMAND}

After logging out and in again, this will update the title of the window to the hostname of the current machine just before the prompt is presented, saving you the trouble of updating the window title if like myself you never use it for anything besides the machine’s hostname.