Differences between version 17 and revision by previous author of PerlOneLiners.
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| Newer page: | version 17 | Last edited on Friday, April 9, 2004 11:32:45 am | by AristotlePagaltzis | Revert |
| Older page: | version 13 | Last edited on Thursday, April 8, 2004 5:11:21 pm | by JohnMcPherson | Revert |
@@ -5,10 +5,11 @@
!! Removing empty lines from a file
perl -ni.bak -e'/\S/ && print' file1 file2
-In Shell:
- for FILE in file1 file2 ; do mv "$F"{,.bak} ; sed
'/
^ *$/d
' <
"$F.bak" > "$F" ; done
+In [
Shell]
:
+
+ for FILE in file1 file2 ; do mv "$F"{,.bak} ; grep
'[[
^ ]
' "$F.bak" > "$F" ; done
!! Collapse consecutive blank lines to a single one
perl -00 -pi.bak -e1 file1 file2
@@ -29,13 +30,36 @@
You can use any punctuation as the separator in an __s///__ command, and if you have backslashes or even need literal slashes in your pattern then doing this can increase clarity.
!! List all currently running processes
- perl -pe 'BEGIN {undef$/;chdir"/proc";@ARGV=sort{$a<=>$b}glob("*/cmdline")}
- $ARGV=~/(\d+)/;print "$1\t";s@\0@ @g;$_.="\n";'
-This prints the process ID and command line of every running process on the system (except some "special" kernel processes that lie about/don't have command lines). You might want such
a command if you suspect
a rootkit or something similar
has been installed
, and you
can't trust your
"ps
" binary
.
+This is useful if you suspect that ps(1) is not reliable, whether due to a RootKit or some other cause. It
prints the process ID and command line of every running process on the system (except some "special" kernel processes that lie about/don't have command lines).
+
+ perl -0777 -pe 'BEGIN { chdir "/proc"; @ARGV = sort { $
a <=> $b } glob("*/cmdline") }
+ $ARGV =~ m!^(\d+)/!; print "$1\t"; s/\/ /g; $_ .= "\n";'
+
+It runs an implicit loop over the __/proc/*/cmdline__ files, by priming __@ARGV__ with a list of files sorted numerically (which needs to be done explicitly using __<=>__ -- the default sort is [ASCII]betical) and then employing the __-p__ switch. __-0777__ forces files to be slurped wholesale. Per file, the digits that lead the filename are printed, followed by a tab. Since a null separates the arguments in these files, all of them are replaced by spaces to make the output printable. Finally, a newline is appended. The print call implicit in the __-p__ switch then takes care of outputting the massaged
command line.
+
+
+!! List all currently running processes, but nicer
+
+See above for what this does. The ''how'' is different, though.
+
+ perl -MFile::Slurp -0le 'for(sort { $
a <=> $b } grep !/\D/, read_dir "/proc")
+ { @ARGV = "/proc/$_/cmdline"; printf " %6g %s\n", $_, join(" ", <>); }'
+
+This time the loop is explicit. Again, there are two parts to the program -- selecting files and doing [IO] on them.
+
+To read the directory, a convenience function is pulled in from the File::Slurp module, loaded using the __-M__ switch. The module
has been part of the core distribution since Perl 5.8.. Reading a directory manually is straightforward but the code would be longer and clumsier.
+
+Selecting the files is pretty simple
, if a little obtuse: it's done by reading the contents of __/proc__, then using __grep !/D/__ to reject any entries that contain non-digit characters from the list. The results,are then sorted numerically, done explicitly using __<=>__ because the default sort is [ASCII]betical. Each entry is then concatenated into a full path
and stuck into __@ARGV__ one by one, from where the __<>__ "diamond operator" will pick it up, auto-open it and read it for us, even autoreporting any errors in a nicely verbose format.
+
+Producing human-readable output is a little more involved, using some switches to abbreviate a bit of magic. The __-__ switch sets the __$/__ variable: here, because it is not followed by a digit, it sets it to a null character. This means that null characters will be regarded as line separators on input. The __-l__ switch has two effects, of which only one is relevant to us: it automatically __chomp()__s lines read using the diamond operator. (The other is to set the __$\__ variable, which we aren't interested in or affected by.)
+
+;: Note that the __-__ and __-l__ switches are order sensitive both in syntax and semantics. We order them for syntax here, because they
can both accept an octal number as input, but we don
't to pass one to either of them (particularly, __-__ will be mistaken for a digit parameter to __-l__ if we turn them around).
+
+Together, these switches effectively mean that we get null terminated lines from files, with the nulls removed on input. So we get the command line arguments listed in a __/proc/*/cmdline__ file as nice list of separate strings. And because __join()__ expects a list, __<>__ returns all
"lines
" (ie command line arguments) at once, which __join()__ then dutifully puts together with spaces between
.
-Basically, this runs a loop over all the files in /proc/*/cmdline, printing the content of those files
(after printing the leading digits in the filename, and replacing null characters with a space
). "$/"
is a special variable used for the end-of-line marker, and it needs to be unset so that even the empty/unreadable files still cause the loop to print out the filename. The __-p__ switch for perl means do the loop, using the input of the rest of the arguments (assumed to be filenames). We cheat and manually assign @ARGV in the BEGIN{..} block. Also, we need to tell sort to use a numeric comparison (<=>) instead of the default string comparison
.
+The printf
(3
) is straightforward
.
----
AddToMe
