How to use while loop to check condition in linux?

Question

I have to do something like this :

while ( condition)

do
 wait 

I have to do this is in linux. While the state of snapshot is pending it should wait.

Content of ec2-describe snapshot is :

SNAPSHOT    snap-c7f3   vol-f6a0    completed   2013-12-04T09:24:50+0000    100%    109030037527    10  2013-12-04: Daily Backup for SaMachine (VolID:vol-f09a0 InstID:i-2604)
SNAPSHOT    snap-c7df9  vol-3f6b    completed   2013-12-04T09:24:54+0000    100%    109030037527    10  2013-12-04: Daily Backup for sa_test_VPC (VolID:vol-3InstID:i-e1c46)

How to do this? How should I use grep and all that ?

#!/bin/bash

# Setting the environmental variables

export EC2_HOME=/opt/ec2/tools
export AWS_ACCESS_KEY_ID=
export AWS_SECRET_ACCESS_KEY=
export SOURCE_REGION="us-west-2"
export DESTINATION_REGION="us-east-1" 


# Set the variable today-date to current date
today_date=`date +%Y-%m-%d`
echo "$today_date"


# Set the variable date_dir to yesterday's date
date_dir=$(date +%Y-%m-%d -d '-1 days')
echo "$date_dir"


#First delete all snapshots older than one day 
#Create a file with all scheduled snapshots

echo "First delete all snapshots older than one day"
echo "Create a file with all scheduled snapshots"
read -rsp $'Press enter to continue...\n'
ec2-describe-snapshots | grep -i "$date_dir">"$EC2_HOME/SnapshotsDOW_$today_date"


#Delete snapshots for older backups

echo "Delete snapshots for older backups"
read -rsp $'Press enter to continue...\n'
for snapshot in $(cat "$EC2_HOME/SnapshotsDOW_$today_date" | awk '{print $2}')
  do
      ec2-delete-snapshot $snapshot
done


#Now create a snapshot for every attached volume to every instance
#Create a file with all attached volumes

echo "Create a file with all attached volumes"
read -rsp $'Press enter to continue...\n'
#ec2-describe-volumes   | grep -i "attached" >"$EC2_HOME/ActiveVolumes_$today_date"


#Create a file with all instances

echo "Create a file with all instances"
read -rsp $'Press enter to continue...\n'
ec2-describe-instances | grep -i "tag" | grep -i "name" >"$EC2_HOME/Instances_$today_date"


#Create snapshots of all attached volumes

echo "Create snapshots of all attached volumes"
read -rsp $'Press enter to continue...\n'
awk '{print $2, $3}' "$EC2_HOME/ActiveVolumes_$today_date" | while read vol_id inst_id; do
    awk '{print $3, $5}' "$EC2_HOME/Instances_$today_date" | while read inst_id2 name; do
        if test "$inst_id" = "$inst_id2"; then
            echo ec2-create-snapshot "$vol_id" -d "$today_date: Daily Backup for $inst_id (VolID:$vol_id InstID:$inst_id)"
             ec2-create-snapshot "$vol_id" -d "$today_date: Daily Backup for $inst_id (VolID:$vol_id InstID:$inst_id)"
        fi
    done
done



#Create a file with all latest snapshots

echo "Create a file with all latest snapshots"
read -rsp $'Press enter to continue...\n'
latestdate=$(ec2-describe-snapshots | grep ^SNAPSHOT | sort -rk 5 | awk '{print substr($5, 1, 10); exit}')
ec2-describe-snapshots | grep "^SNAPSHOT.*$latestdate" > "$EC2_HOME/SnapshotsLatest_$today_date"


#Copy the snapshot across multiple regions.

echo "Copy the snapshot across multiple regions."
read -rsp $'Press enter to continue...\n'
for snapshot in $(cat "$EC2_HOME/SnapshotsLatest_$today_date" | awk '{print $2}') 
do 
  ec2-copy-snapshot -r $SOURCE_REGION -s $snapshot -region $DESTINATION_REGION
done


#Send the latest snapshot details to user using mail command.
echo "Send the latest snapshot details to user using mail command"
read -rsp $'Press enter to continue...\n'
mail -s 'VM Snapshot $today_date' jp.com <  "$EC2_HOME/SnapshotsLatest_$today_date"

read -rsp $'Press enter to exit...\n'

Answer 1

until ec2-describe-snapshots | grep -q ' completed '
do
    echo "snapshot is pending, still waiting..."
    sleep 10
done
echo "snapshot completed."

If you need all the snapshots to be completed, you could use instead:

until ! ec2-describe-snapshots | grep -q ' pending '
do
    echo "snapshot is pending, still waiting..."
    sleep 10
done
echo "snapshot completed."

UPDATE

And, to handle also no output case:

until [ -n "`ec2-describe-snapshots`" ]
do
    echo "snapshot not yet started, still waiting..."
    sleep 10
done
until ! ec2-describe-snapshots | grep -q ' pending '
do
    echo "snapshot is pending, still waiting..."
    sleep 10
done
echo "snapshot completed."

Answer 2

Writing that while loop is as simple as this:

while ec2-describe-snapshots | grep -q '^SNAPSHOT.*pending'
do
    echo "There are pending SNAPSHOTs, waiting..."
    sleep 10
done
echo "No pending SNAPSHOTs."

Answer 3

If you can put it in a shellscript, here's how i'd do it.

Edit

So after reading your current code this is what i would do. You'll have to test it yourself though.

# This is where you're creating your snapshots...

# Setup the command.
command=`ec2-describe-snapshots | grep pending | wc -l`

# Check if we have any pending snapshots at all.
if [ $command == "0" ]
then
        echo "No snapshots are pending."
        ec2-describe-snapshots
else
        # Wait for the snapshot to finish.
        while [ $command != "0" ]
        do
                # Communicate that we're waiting.
                echo "There are $command snapshots waiting for completion."
                sleep 5

                # Re run the command.
                command=`ec2-describe-snapshots | grep pending | wc -l`
        done

        # Snapshot has finished.
        echo "Snapshots are finished."
fi       

# This is where you're writing snapshots to file...

Old script kept for reference

#!/bin/sh

waitForSnapshot() {
        # Make sure we actually passed a snapshot to grep for.
        if [ -n "$1" ]
        then
                # Setup the command.
                command=`ec2-describe-snapshots | grep $1 | awk '{print $4}'`

                # Wait for the snapshot to finish.
                while [ $command != "completed" ]
                do
                        sleep 1

                        # Re run the command.
                        command=`ec2-describe-snapshots | grep $1 | awk '{print $4}'`
                done

                # Snapshot has finished.
                echo "Snapshot '$1' has finished."
        else
                echo "No snapshot was passed to us."
        fi

}

waitForSnapshot "snap-c7f3"