Unix Shell scripting

This post was badly required, just for me. Every time I have a write a script, I google. Its difficult to remember the various options of various commands. And most of the time its search, extract or replace. So here it goes

1. To split a string and extract a value
value=$(echo $line | awk '{split($0, splitValues, " ");  print splitValues[1] }')

• Input to split is the value in the variable line.
• split command splits the string, executing the regular expression within quotes and stores in an array splitValues.
• print splitValues[1] returns the first element in array splitValues
• If you dont want to store the value then  echo $line | awk '{split($0, splitValues, " ");  print splitValues[1] }' will print it on the console

2. Find a matching pattern in a file and extract them to another file
grep -n 'match this' test.log > result.log

• grep searches the complete file test.log for the string mentioned in quotes
• all the lines which matches the pattern are extracted to result.log
• -n prefixes the line number of every match (-n is optional)

3.  Print last few lines from a file
tail -n

• This command extract the number of lines of the specified file and prints it on the console
• The output can always be redirected to a file tail -10 output.txt > subset.txt

4. Executing an sql in shell script - There are few steps to achieve this

• Write a sql and store it in a file, say SelectQuery.sql
• The shell script to connect to database and execute the query in SelectQuery.sql looks something like this

 #!/bin/bash
sqlplus -s -l /@  << EOF >> output.txt
@SelectQuery.sql
exit
EOF

while read line
do
    echo $line
done <  output.txt

errorCode=$?    # checks if the last operation (sqlplus) was completed successfully or not

if [ ${errorCode} -ne 0 ]
then
echo "SQLPlus was unable to connect the DB with the supplied credentials"
else
echo  "SQLPlus was connected successfully"
fi

So what does this script do ?

• Connects to database using sqlplus command 
• output.txt is where the results of the query will be stored
• @SelectQuery.sql actually executes the query
• The while loop, iterates through each lines in the output.txt and prints them on console
• A small check to display an errors at the end