Programming in Go

Regular Expressions

The fields can be processed with the fmt.Scanf function, which works in the same way as the corresponding function in C: Format strings specify the format of the read row and the data type. Alternatively – but possibly a bit more slowly – you can do the same thing with regular expressions. To this end, Go offers the regexp package [3], which implements the RE2 regular expression syntax [4].

The functions provided by the module are a bit confusing but do follow a system. First, you'll see two variants of each function: one that processes a byte array and one with the word String in the name, which deals with strings. Then, you'll see functions that return the string found once only, or more than once (All). If you use multiple search strings in a regular expression and assign variables to the strings you find, you will want to use a function with Submatch in its name.

In this example, you need the FindStringSubmatch function because the regex pattern is designed so that it can match only once anyway, while storing all the finds in one array. The byte array from ReadFile is converted to a string by typecasting. If you were to use the byte array variants of the regular expression functions, you would then continue to work with byte arrays or convert them.

Several functions are available to compile the search pattern: Compile, CompilePOSIX, MustCompile, and MustCompilePOSIX. Why this is so is not completely apparent, because the same thing could have been implemented with a single function that processes the corresponding parameters. To prevent the regular expression being constantly recompiled, you can define it as a top-level variable. The corresponding code is shown in Listing 1.

Listing 1

Regular Expressions

 

Line 14 of the code creates a new ProcData object and assigns the first two submatches to its attributes pid and name. In line 9, the regular expression is contained in backticks (`) because a backslash (\), is regarded as an escape character in strings in single or double quotes. You would otherwise need to type another backslash in front of each one you really needed. In the backtick environment, you can save yourself the trouble.

The procedure for the /proc/PID/status file, which you also need to read, is similar because the user ID of the process does not appear in the stat file. However, the status file is a little more difficult to parse because it contains many individual lines. You could process the file line by line or use a regular expression in multiline mode enabled by the m switch at the start of the regex. This alone, however, was not quite enough; I also needed the s switch, which ensures that the dot meta-variable (.) in the regular expression also includes the newline character (\n). The regex for reading the user and group ID from the status file, thus looks like this:

(?sm)^Uid:\t(\d+).*^Gid:\t(\d+)

The UID is now assigned to a field in the ProcData object.

The remaining task is to determine the UID for the user name – for example, by parsing the passwd file. However, Go can save you this work because it has the user.LookupId function for precisely this task. As a result, it returns a user object containing the user name, among other things. All told, the procedure looks like this:

user, _ := user.LookupId(procData.uid)
procData.user = user.Username

One option of the lap tool is to determine whether to output the name or UID. For easy implementation of command-line options, Go includes the flag package. Like everything else in Go, flags are typed and are available in various flavors (e.g., integer, string, and Boolean). A Boolean switch is defined by the line:

var realname = flag.Bool("r", false, "show real user name")
flag.Parse()

The first parameter specifies the name of the switch, followed by the default configuration and the explanatory text. Later in the program, you can query the variable using if(realname).

When called with -r, lap then returns the user name; otherwise, it returns the UID (Figure 3). The main features of the program are shown in Listing 2; the complete listing is available online [5].

Listing 2

lap.go

 

Figure 3: The result of all the hard work: The lap process tool displaying running processes and user names.

Missing Bits

With relatively little effort, a small Go tool has been developed to identify the processes on a Linux system and display them at the command line. However, you could improve this by adding a cache for username lookup, for example. The run-time information of the processes (i.e., the start times, etc.) is also missing and could be added to the tool.

Infos

  1. "Go Programming Language" by Oliver Frommel, ADMIN, issue 11, 2013, pg. 90
  2. The /proc filesystem: https://www.kernel.org/doc/Documentation/filesystems/proc.txt
  3. Go regexp package: http://golang.org/pkg/regexp/
  4. RE2 syntax: https://code.google.com/p/re2/wiki/Syntax
  5. Listings: ftp://ftp.linux-magazin.com/pub/listings/magazine/162

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