Using mirrors in SED

It took me long to discover mirrors in SED. That’s a feature that may help you a lot in some replacing problems. Mirrors are denoted by \1, \2, \3, .. \9 and are used to cut a part of the input string to use it in the replacement string, like a temporary variable. For use mirrors in SED, you need to use extended regexp, flag -r. Let’s see some applications:

Chaging a data:

$ echo 12/31/2004 | sed -r 's@([0-9][0-9])/([0-9]{2})/([0-9]{4})@\1.\2.\3@'
$ echo 12/31/2004 | sed -r 's@([0-9][0-9])/([0-9]{2})/([0-9]{4})@\2.\1.\3@'
$ echo 12/31/2004 | sed -r 's@([0-9][0-9])/([0-9]{2})/([0-9]{4})@\2-\1-\3@'

Inverting field order:

$ echo abcdefg | sed -r 's/(a)(b)(c)/\3:\2:\1/'
$ echo abcdefg | sed -r 's/(a)(b)(c).*/\1:\2:\3/'
$ echo abcdefg | sed -r 's/(a)(b)(c).*/\3:\2:\1/'

Read more:
http://aurelio.net/curso/sucesu/sucesu-seder-prompt.html

See ya!

How to generate a bash script with an embeeded tar.gz (self-extract)

Consider that you need to perform a routine in a remote server, where you need to decompress a tar.tz and execute a list of commands on this data. One alternative is send the tar.gz file to the remote server throught a ftp or scp and then log in the remote server and run a shell script or run manually a list of commands. Recall Java JRE setup, they use script.bin that comes with an embeeded tar.gz, which is self-extracted in the beginning of script execution. To build the self-extraction script I follow a tutorial published by Stuart Wells, which consists in four steps:

1) Create/identify a tar.gz file that you wish to become self extracting.

2) Create the self extracting script. A sample script is shown below:

> cat extract.sh
#!/bin/bash
echo "Extracting file into `pwd`"
# searches for the line number where finish the script and start the tar.gz
SKIP=`awk '/^__TARFILE_FOLLOWS__/ { print NR + 1; exit 0; }' $0`
#remember our file name
THIS=`pwd`/$0
# take the tarfile and pipe it into tar
tail -n +$SKIP $THIS | tar -xz
# Any script here will happen after the tar file extract.
echo "Finished"
exit 0
# NOTE: Don't place any newline characters after the last line below.
__TARFILE_FOLLOWS__

3) Concatenate The script and the tar file together.

> cat extract.sh example.tar.gz > example.sh
> chmod +x example.sh

4) Now test in another directory.

> cp example.sh /tmp
> cd /tmp
> ./example.sh

See ya!