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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
Old 10-24-2007
Hi
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
 

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PYP(1)							      General Commands Manual							    PYP(1)

NAME
pyp - The Pyed Piper: A Modern Python Alternative to awk, sed and Other Unix Text Manipulation Utilities SYNOPSIS
pyp [options] files ... DESCRIPTION
pyp, the Pyed Piper, is a command line tool for text manipulation. It is similar to awk and sed in functionality, but its subcommands are Python based, and thus more familiar to many programmers. It can operate both on a per-line base and on the complete input stream. Different features can be pipelined in a single command by using the pipe character familiar from shell commands. pyp backs up its input for reruns with modified commands, and can save commands as macros. On the downside, the rerun feature makes it unsuitable for continuous pipe operation. OPTIONS
These programs follow the usual GNU command line syntax, with long options starting with two dashes (`-'). A summary of options is included below. For a complete description, use --manual. -h, --help Show this help message and exit. -m, --manual Prints out extended help. -l, --macro_list Lists all available macros. -s MACRO_SAVE_NAME, --macro_save=MACRO_SAVE_NAME Saves current command as macro. use "#" for adding comments EXAMPLE: pyp -s "great_macro # prints first letter" "p[1]". -f MACRO_FIND_NAME, --macro_find=MACRO_FIND_NAME Searches for macros with keyword or user name. -d MACRO_DELETE_NAME, --macro_delete=MACRO_DELETE_NAME Deletes specified public macro. -g, --macro_group Specify group macros for save and delete; default is user. -t TEXT_FILE, --text_file=TEXT_FILE Specify text file to load. For advanced users, you should typically cat a file into pyp. -x, --execute Execute all commands. -c, --turn_off_color Prints raw, uncolored output. -u, --unmodified_config Prints out generic PypCustom.py config file. -b BLANK_INPUTS, --blank_inputs=BLANK_INPUTS Generate this number of blank input lines; useful for generating numbered lists with variable 'n'. -n, --no_input Use with command that generates output with no input; same as --dummy_input 1. -k, --keep_false Print blank lines for lines that test as False. default is to filter out False lines from the output. -r, --rerun Rerun based on automatically cached data from the last run. Use this after executing "pyp", pasting input into the shell, and hitting CTRL-D. SEE ALSO
awk(1), grep(1), sed(1). AUTHOR
pyp was written by Toby Rosen <tobyrosen@gmail.com>. This manual page was written by Khalid El Fathi <khalid@elfathi.fr>, for the Debian project (and may be used by others). March 19, 2012 PYP(1)
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