vi comment and uncomment multiple lines

Comment multiple lines

1. Press ESC
2. Go to the starting line, press ctrl+v to enable visual block mode
3. Use down arrow or k key to select multiple lines
4. Press shift+I to enable insert mode
5. Press # to add comment to first line, then press ESC, then wait for 1 second to get # added to all lines

Another option is:

1. Press ESC
2. Go to the starting line, press ctrl+v to enable visual block mode
3. Use down arrow or k key to select multiple lines
4. Press shift+ : to do search and replace ('<,'> will be automatically added)
5. Type  s/^/#/ to add # to comment out selected lines

Uncomment multiple lines

1. Press ctrl+v to enable visual block mode
2. Move down to select lines
3. Press x to uncomment selected lines

Copy folder from local to remote or vice versa

Option 1: scp -r
To copy all from Local Location to Remote Location (Upload)
scp -r /path/to/file username@hostname:/path/to/destination

To copy all from Remote Location to Local Location (Download)
scp -r username@hostname:/path/to/file /path/to/destination

Custom Port where xxxx is custom port number
scp -r -p xxxx username@hostname:/path/to/file /path/to/destination

Copy on current directory from Remote to Local
scp -r username@hostname:/path/to/file .


Option 2: zip to file then scp
Better to first compress catalog on remote server:
tar czfP backup.tar.gz /path/to/catalog

Secondly, download from remote:
scp user@your.server.example.com:/path/to/backup.tar.gz .

At the end, extract the files:
tar -xzvf backup.tar.gz

Option 3: sftp
$ sftp user_remote@remote_ip
sftp> cd /path/to/remoteDir
sftp> get -r remoteDir

Shell script error: bad interpreter: No such file or directory

It is highly likely that you created this file with a windows editor, which will place a <cr><lf> at the end of each line. This is the standard under dos / windows. However, under Unix / linux, the standard is to just put a <lf> at the end of the line.

Linux is now looking for a file called /bin/bash<cr> to interpret the file, where <cr> is a carriage return character, which is a valid file character under linux. Such a file doesn't exist. Hence the error.

http://stackoverflow.com/questions/2841593/bash-script-bad-interpreter

Sublime Text 2 -> View -> Line Endings -> Unix

cannot open '/etc/mail/local-host-names': World writable directory

What is World-writable?
A world-writable file or directory is one that anyone can alter. For directories this means that files can be created or deleted within it by anyone. Being world-writable is almost always a security risk.

Problem:
Get below error when try to start sendmail (/etc/init.d/sendmail start)
/etc/mail/sendmail.cf: line 94: fileclass: cannot open '/etc/mail/local-host-names': World writable directory

Solution:

1. Grant w to /etc, /etc/mail, /etc/mail/* => not work
2. Manually edit file sendmail.cf

replace line Fw/etc/mail/local-host-names with line
Fw-o /etc/mail/local-host-names

identically (if you got the same error for trusted-users)
replace line Ft/etc/mail/trusted-users with line
Ft-o /etc/mail/trusted-users

Shell script to monitor disk space

Motivation:
Space full caused db transaction hang issue last week, and this week it caused javax.jms.JMSException: No space left on device, herein ActiveMQ cannot write message to its queue. I decide to write a shell script to watch the disk space.

Steps:

1. Google for similar script and info
2. Write the shell script 
3. Start sendmail for email alert
4. Add script to cronjob for auto monitoring

Commands:

1. /etc/init.d/sendmail => Usage: /etc/init.d/sendmail {start|stop|restart|condrestart|status}
2. /etc/init.d/crond => Usage: /etc/init.d/crond {start|stop|status|reload|restart|condrestart}

Two options to add script to cronjob:

1. crontab -e
2. Put shell script to /etc/cron.* folder (cron.daily/   cron.hourly/  cron.monthly/ cron.weekly/)

Script:

# http://www.cyberciti.biz/tips/shell-script-to-watch-the-disk-space.html
# set admin email for alert
ADMIN_EMAIL="admin@sample.com"
# set alert level
ALERT_LEVEL=90
# set log folder to get deleted
LOG_FOLDER=/opt/logs/*

df -HP | grep -vE '^Filesystem|tmpfs|cdrom' | awk '{ print $5 " " $1 }' | while read output;
do
  echo $output
  used=$(echo $output | awk '{ print $1}' | cut -d'%' -f1 )
  partition=$(echo $output | awk '{ print $2 }' )
  if [ $used -ge $ALERT_LEVEL ]; then
    echo "Running out of space \"$partition ($used%)\" on $(hostname) as on $(date)" |
    mail -s "Alert: Running out of disk space - $used% used" $ADMIN_EMAIL

              echo "Purging log files ..."
              find $LOG_FOLDER -mtime +2 -exec rm -f {} \;

              echo "DONE"   

  fi
done

Why use df - HP?

linux01:root > df -H
Filesystem             Size   Used  Avail Use% Mounted on
/dev/sda2              8.0G   2.7G   5.0G  35% /
192.168.253.131:/u001/ReleaseArchive/
                       289G   203G    78G  73% /jdrive

linux01:root > df -H -t ext3
Filesystem             Size   Used  Avail Use% Mounted on
/dev/sda2              8.0G   2.7G   5.0G  35% /

linux01:root > df -HP
Filesystem             Size   Used  Avail Use% Mounted on
/dev/sda2              8.0G   2.7G   5.0G  35% /
192.168.253.131:/u001/ReleaseArchive/   289G   203G    78G  73% /jdrive

Reference:
http://www.cyberciti.biz/tips/shell-script-to-watch-the-disk-space.html