Default grep options

When you’re searching a set of version-controlled files for a string with grep, particularly if it’s a recursive search, it can get very annoying to be presented with swathes of results from the internals of the hidden version control directories like .svn or .git, or include metadata you’re unlikely to have wanted in files like .gitmodules.

GNU grep uses an environment variable named GREP_OPTIONS to define a set of options that are always applied to every call to grep. This comes in handy when exported in your .bashrc file to set a “standard” grep environment for your interactive shell. Here’s an example of a definition of GREP_OPTIONS that excludes a lot of patterns which you’d very rarely if ever want to search with grep:

for pattern in .cvs .git .hg .svn; do
    GREP_OPTIONS="$GREP_OPTIONS --exclude-dir=$pattern

Note that --exclude-dir is a relatively recent addition to the options for GNU grep, but it should only be missing on very legacy GNU/Linux machines by now. If you want to keep your .bashrc file compatible, you could apply a little extra hackery to make sure the option is available before you set it up to be used:

if grep --help | grep -- --exclude-dir &>/dev/null; then
    for pattern in .cvs .git .hg .svn; do
        GREP_OPTIONS="$GREP_OPTIONS --exclude-dir=$pattern"

Similarly, you can ignore single files with --exclude. There’s also --exclude-from=FILE if your list of excluded patterns starts getting too long.

Other useful options available in GNU grep that you might wish to add to this environment variable include:

  • --color — On appropriate terminal types, highlight the pattern matches in output, among other color changes that make results more readable
  • -s — Suppresses error messages about files not existing or being unreadable; helps if you find this behaviour more annoying than useful.
  • -E, -F, or -P — Pick a favourite “mode” for grep; devotees of PCRE may find adding -P for grep‘s experimental PCRE support makes grep behave in a much more pleasing way, even though it’s described in the manual as being experimental and incomplete

If you don’t want to use GREP_OPTIONS, you could instead simply set up an alias:

alias grep='grep --exclude-dir=.git'

You may actually prefer this method as it’s essentially functionally equivalent, but if you do it this way, when you want to call grep without your standard set of options, you only have to prepend a backslash to its call:

$ \grep pattern file

Commenter Andy Pearce also points out that using this method can avoid some build problems where GREP_OPTIONS would interfere.

Of course, you could solve a lot of these problems simply by using ack … but that’s another post.

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.