SED help (remove line::parse again::add line) Post: 302142125

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Top Forums Shell Programming and Scripting SED help (remove line::parse again::add line) Post 302142125 by drl on Wednesday 24th of October 2007 12:36:25 PM

10-24-2007

Registered User

Quote:

Originally Posted by bakunin

The reason is: sed will read the first line of the input file (which is "file"), apply any rules that might be applicable and then write the result (the changed or not changed line) to <stdout>. This <stdout> is at this moment pointing to "file" and this is why after the first line the file will contain nothing more than exactly that first line - so sed encounters the EOF and thinks it is finished.

I would have explained this behaviour somewhat differently: when the shell begins to process a command line, it will literally remove the characters that involve re-direction. In dealing with re-direction, the shell will "zero" a file that is being re-directed to. If there was data on the file, it will have been erased before the command is executed. The result is that the command -- in this case sed -- will never see any input, because it already has been destroyed. The first read will encounter EOF.

Most of us have fallen into this operational trap. For example, we often want to sort a file onto itself -- just re-ordering data, neither gaining or losing data. So sort has an option to write the output to the input file, but one must use "-o file", and not "> file". The GNU sed allows a similar construction with "in-place" editing, but what actually happens is that sed writes to a temporary file and then renames it at the end. You can see this if you display the inode number (ls -li file) before and after the operation. You'll see that they are different.

The flow-chart of shell operations is displayed in some books, for example, O'Reilly's Learning the bash shell, 2nd, page 177 ff ... cheers, drl

drl

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cgcreate

CGCREATE(1)							 libcgroup Manual						       CGCREATE(1)

NAME

       cgcreate - create new cgroup(s)

SYNOPSIS

       cgcreate [-h] [-s] [-t <tuid>:<tgid>] [-a <agid>:<auid>] [-f mode] [-d mode] -g <controllers>:<path> [-g ...]

DESCRIPTION

       The command creates new cgroup(s) defined by the options -g.

       -a <agid>:<auid>
	      defines  the  name of the user and the group which own the rest of the defined control group's files. These users are allowed to set
	      subsystem parameters and create subgroups.  The default value is the same as has the parent cgroup.

       -d, --dperm=mode
	      sets the permissions of a control groups directory.  The permissions needs to be specified as octal numbers e.g.	-d 775.

       -f, --fperm=mode
	      sets the permissions of the control groups parameters.  The permissions needs to be specified as octal numbers e.g.   -f	775.   The
	      value  is  not  used as given because the current owner's permissions are used as an umask (so 777 will set group and others permis-
	      sions to the owners permissions).

       -g <controllers>:<path>
	      defines control groups to be added.  controllers is a list of controllers and path is the relative path to  control  groups  in  the
	      given controllers list. This option can be specified multiple times.

       -h, --help
	      display this help and exit

       -s, --tperm=mode
	      sets  the  permissions  of  the control group tasks file.  The permissions needs to be specified as octal numbers e.g.  -f 775.  The
	      value is not used as given because the current owner's permissions are used as an umask (so 777 will set group  and  others  permis-
	      sions to the owners permissions).

       -t <tuid>:<tgid>
	      defines  the  name of the user and the group, which owns tasks file of the defined control group. I.e. this user and members of this
	      group have write access to the file.  The default value is the same as has the parent cgroup.

FILES

SEE ALSO

       cgrules.conf (5) cgexec (1) cgclassify (1)

Linux								    2009-03-15							       CGCREATE(1)