General

xargs command

To run commands on standard output of some command instead of using for loop.

 # ls | xargs -t -n1 ls -l

Use -i or -I {} for inserting filenames in the middle of command line. For example, To rename all the files in a directory with an .old extension

 # ls | xargs -t -i mv {} {}.old

sudo

The default configuration file for sudo is /etc/sudoers. Do not directly edit the /etc/sudoers file. Always use visudo command. If visudo command opens some other editor other than vi, we can correct it setting EDITOR variable. (eg. export EDITOR=vi)

There are four kinds of aliases in sudoers file

User_Alias
Runas_Alias
Host_Alias
Cmnd_Alias

Each alias definition is of the form

 Alias_Type NAME = item1, item2, ...

  user or group      hosts or      
  or (Useralias)     (hostalias)= (run-as alias)   (command alias)

Converting time to epoch and viceversa

To get the current time in epoch seconds (On HP-UX and Linux)

 # date +%s
 1214425649

 # perl -le "print (time)"
 1214426325

Convert from human readable date to epoch (On Linux)

 # date +%s -d"Jan 1, 1980 00:00:01"
 315561601

 # perl -e perl -e "use Time::Local; print timelocal($sec, $min, $hours, $mday, $mon, $year)"
 # perl -e "use Time::Local; print timelocal(20, 56, 16, 25, 05, 2008)"
 1214438180

To convert from epoch to Human readable format (On Linux)

 # date -d @1190000000
 Sun Sep 16 20:33:20 PDT 2007
        or
 perl -e "print scalar(localtime(1190000000))"
 Sun Sep 16 20:33:20 2007