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It is possible to hide a rar archive inside a png image file and then retrieve the files from this image.

cat picture.png archive.rar > hidden_archive_in_pic.png

This can also be done on Windows:

copy picture.png + archive.rar hidden_archive_in_pic.png

When you want to retrieve the hidden files, download the image, rename to .rar and extract.


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If, like me, you often make mistakes on the command line, try using the history shortcut '^^' to repeat the last command with changes.

For example:

$ cat fiel
cat: fiel: No such file or directory
$ ^el^le
cat file


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If you want to surround the output of a command by a header or a footer try the following:

$ command | cat headerFile - footerFile


For example
$ ls *txt | cat header - footer
Here is a list of files:
1.txt
2.txt
End of file list


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You can view unprintable and non ASCII characters in a file with 'cat -v -t -e'

Example:

$ cat myfile
Hello,  World!    
$ cat -v -t -e myfile 
Hello, ^IWorld!    $


In this file there is a tab between the two words and 4 spaces at the end of the line that would not have been visible with a plain cat.


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Zero out files leaving directory and file sets in place - used for archival purposes.
/// bash script file: cdn (located in /usr/local/bin)
#!/bin/bash
#
# Cataboligne - zero a single file passed in argument 1, and make a log entry
#
cat /dev/null > "$1"
echo "zero $1" >> $HOME/.zero-log
/// end bash script

command line:
find  -wholename '*/*.[mM][pP][32cC]' -exec echo {}\; -exec cdn {} \;


This example zeros all mp2/mp3/mpc of any case anywhere under the current directory + sub directories and tells you what got zapped.

Do the same thing for all files in a directory from a file manager (such as rox) menu shortcut.
/// bash script file: xt-zerofiles (located in /usr/local/bin)
#!/bin/bash
#
# Cataboligne - zero all files in a directory structure with double logging
# and verify, called from rox sub menu
#
cd "$@"
echo "" > /tmp/.cont
xterm -geometry 70x1 -title 'Zero Caution!' -e 'read -p "zero files in $PWD: Y/N? " RSP;echo $RSP > /tmp/.cont'
RSP2=`cat /tmp/.cont`
if [[ "$RSP2" != "Y" && "$RSP2" != "Yes" && "$RSP2" != "y" ]]; then exit; fi;
echo >> $HOME/.zero-log-summary
echo >> $HOME/.zero-log        
echo $(date) " zero fn() = $PWD" >> $HOME/.zero-log-summary
echo "------------------------------" >> $HOME/.zero-log-summary
echo $(date) " zero fn() = $PWD" >> $HOME/.zero-log        
echo "------------------------------" >> $HOME/.zero-log        
find -type f -exec cdn {} \;
rm -f /tmp/.cont
echo >> $HOME/.zero-log-summary
echo >> $HOME/.zero-log
/// end bash script

The method of calling this script depends on the file manager. With rox you right click on a file (such as a directory) and select "Customize menu" on the selected file sub menu. A rox window pops open with all the menu items for that file type and you can then open rox /usr/local/bin and drag and drop a link for xt-zerofiles.
I recommend a link instead of a copy so if you update the script you dont have to figure out where you copied it. The custom menu is my single favorite rox feature.

The script pops open a small bash window to ask for verification due to the risk of running such a command arbitrarily. And yes, I keep two logs there, you could reduce it to the $HOME/.zero-log or even have no logging.

/// Cataboligne


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cat file.txt | sort -R | tail -1

pokes a random line from a file


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Tired to get your mailbox full of cron information messages?
Eg.: some programs (pg_dump, /etc/init.d/xxx, ...) output informational messages on stderr, but you can't close stderr because it may also contain failure informations.

#!/bin/bash

LOG="/tmp/myscript-`date +%s`.tmp" IGNORE='^(Username: Password:|)[[:space:]]*$' # message to ignore, for demonstration only exec 3<&2; exec 2>$LOG # duplicate stderr for later use; redirect stderr to $LOG file
# insert your code (eg.: echo -e "login\npassword"|pg_dump -u user > file.dump)
exec 2<&3 # restore stderr # if $LOG file is not empty and contain anything else than what should be ignored, output it to stderr (for demonstration purpose) [ -s $LOG ] && egrep -vq $IGNORE $LOG && egrep -v $IGNORE $LOG >&2 rm -f $LOG


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CASE: You've relocated Subversion and Trac repositories to another machine/directories. You don't want to edit n+1 trac.ini files.

You have for example:
/repos/svn-myprojects/my-first-project
/repos/trac-myprojects/my-first-project

/repos/svn-myprojects/my-second-project
/repos/trac-myprojects/my-second-project

...etc...

Change ALL trac.ini repository_dir settings:

trac-mass-repodir-edit.sh:

#!/bin/bash TRACSPATH=$1 REPOPATH=$2 for i in $( find $TRACSPATH -maxdepth 1 -type d | grep -v "^$TRACSPATH\$" ); do BN=`basename $i` INIPATH=$i/conf/trac.ini TMP=$i/conf/trac.ini-temp # Replace repository_dir cat $INIPATH | perl -pe "s@repository_dir = .*@repository_dir = $REPOPATH/$BN@i" > $TMP mv $INIPATH $INIPATH-old && mv $TMP $INIPATH && rm $INIPATH-old done


Usage:
./trac-mass-repodir-edit.sh /repos/trac-myprojects /repos/svn-myprojects


Then you want to resync and upgrade all existing Tracs:

trac-mass-upgrade.sh:
#!/bin/bash

TRACSPATH=$1
REPOPATH=$2

for i in $( find $TRACSPATH -maxdepth 1 -type d | grep -v "^$TRACSPATH\$" ); do
  BN=`basename $i`
  # SVN directory exists
  if [ -d $REPOPATH/$BN ]; then
    echo "Processinc Trac: $BN.."
    trac-admin $i resync
    trac-admin $i upgrade
  fi
done


Usage:
./trac-mass-upgrade.sh /repos/trac-myprojects /repos/svn-myprojects


Now you have everything in order.


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Print a file until a regular expression is matched.
cat file.txt | perl -pe "exit if(/Last line we want/)"


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This short script reads a file line-by-line ( whith whitespace characters too ) and outputs to STDOUT.

#!/bin/bash
FILE='/path/to/filename'
for linecount in `seq $(cat $FILE | wc -l)`; do
line=`head -n$linecount $FILE | tail -1`
echo $line
done


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