Command Line – Understanding Locate Command Regex

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To use locate command with regex , do we need to enclose the pattern in quotes along with passing --regex option ? If yes, then what do the following mean –

a) locate --regex file* ? Here regex will happen or shell globbing ?

b) locate 'file*' ? Will locate do regex search even though we did not passed --regex ?

c) locate file* // I understand shell globbing will happen

d) locate --regex 'file*' // I understand regex search will happen in Database file

Best Answer

You type the command at the shell prompt. The shell processes what you typed, which includes globbing, substituting variables, substituting $() and so on. After processing what you typed, the shell executes the command.

Quotes are needed if a string contains characters that are special to the shell, such as spaces or asterisks, but you don't want the shell to process them. You will get away without quoting an asterisk if there are no matching files in the current directory, but it's good practice to quote it anyway.

It is important to understand that the --regex option has no effect on the shell's actions. First, the shell processes the command that you type. locate gets the result of that processing.

a) If there are files in the current directory that match file*, the shell will replace file* with the list of those files before calling locate. If there is no match, the shell won't touch file*, and locate looks for files that are named file, filee, fileee and so on. In short, the shell attempts globbing, then locate performs a regular expression search if the shell's globbing results in correct syntax.

b) The quotes tell the shell to leave the asterisk alone. locate will look for files that start with file. No regex search.

c) The shell attempts globbing as in a). If there is no match, locate will look for files that start with file. No regex search.

d) The shell leaves the expression alone. locate will perform a regex search and look for files that are named file, filee, fileee and so on.

Related Solutions

Number of Backslashes Needed for Escaping Regex Backslash on Command-Line

For the unquoted example, each \\ pair passes one backslash to grep, so 4 backslashes pass two to grep, which translates to a single backslash. 6 backslashes pass three to grep, translating to one backslash and one \c, which is equal to c. One additional backslash does not change anything, because it is translated \c -> c by the shell. Eight backslashes in the shell are four in grep, translated to two, so this does not match anymore.

For the example in double quotes, note what follows your second quote from the bash manpage:

The backslash retains its special meaning only when followed by one of the following characters: $, `, ", \, or newline.

I.e. when you give an odd number of backslashes, the sequence ends in \c, which would be equal to c in the unquoted case, but when quoted, the backslash looses its special meaning, so \c is passed to grep. That is why the range of "possible" backslashes (i.e. those that make up a pattern matching your example file) slides down by one.

Bash Commands – How to Act on the Results of the Locate Command

Yes, you can use xargs for this.

For example a simple:

$ locate commands.cfg | xargs grep check_dns

(When grep sees multiple files it searches in each one and enables filename printing along matches.)

Or you can explicitly enable filename printing via:

$ locate commands.cfg | xargs grep -H check_dns

(Just in case one grep is called only with 1 argument by xargs)

For programs that only accept one filename argument (unlike grep) you can restrict the number of supplied arguments like this:

$ locate commands.cfg | xargs -n1 grep check_dns

That does not print the names of files where matched lines are from.

The result is equivalent to:

$ locate commands.cfg | xargs grep -h check_dns

With a modern locate/xargs you can also protect against whitespace issues:

$ locate -0 commands.cfg | xargs -0 grep -H check_dns

(By default whitespace separates input of xargs - which is of course a problem when your filenames contain whitespace ...)

Best Answer

Related Solutions

Number of Backslashes Needed for Escaping Regex Backslash on Command-Line

Bash Commands – How to Act on the Results of the Locate Command

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