As per the en.wikibooks.org:

Bash is a “Unix shell”: a command-line interface for interacting with the operating system. It has the ability to run an entire script of commands, known as a “Bash shell script”.

I recently started to create UNIX / LINUX Bash Shell script for enhancing my PostgreSQL DBA Work. You can create a Bash shell script and can connect PostgreSQL using psql.

In this post, I am sharing a sample bash shell script to execute psql commands.

Open a new file in vi editor:

[root@dbrnd anvesh]$ vi helloworld.bash

Press [i] key to get an insert prompt:

Copy below sample Bash Shell Script and Paste into vi editor:

#!/bin/bash

###################################################
# First Bash Shell script to execute psql command 
###################################################

#Set the value of variable
database="database_name"

#Execute few psql commands: 
#Note: you can also add -h hostname -U username in the below commands. 

psql -d $database -c "CREATE TABLE public.tbl_students(rno integer, name character varying)"
psql -d $database -c "INSERT INTO public.tbl_students VALUES (1,'Anvesh'),(2,'Neevan'),(3,'Roy'),(4,'Martin'),(5,'Jenny')"
psql -d $database -c "SELECT *FROM public.tbl_students"

#Assign table count to variable
TableCount=$(psql -d $database -t -c "select count(1) from public.tbl_students")

#Print the value of variable
echo "Total table records count....:"$TableCount

Press [Esc] key to get a command prompt and execute below command:
To save and exit vi editor.

:wq

Give permission to the file:

[root@dbrnd anvesh]$ chmod +x helloworld.bash

Execute this file:

[root@dbrnd anvesh]$ ./helloworld.bash

You can find below result:

rno |  name
-----+--------
   1 | Anvesh
   5 | Jenny
   2 | Neevan
   3 | Roy
   4 | Martin
(5 rows)

Total table records count....: 5

Recently, I started to configure few of my DBA shell scripts in Linux Crontab.

Yesterday, I shared an important article on skip the execution of your shell script if it is running already.

If you forget to put the first line of your shell script something like “#!/bin/sh” and bash shell script something like “#!/bin/bash”, you cannot get the name of your running shell script.

It’s called a shebang and tells the parent shell which interpreter should be used to execute the script.

When you are running shell script in Linux, the operating system will look at the first few bytes of the file to decide the format of the program and how to execute it.

If you don’t use “#!/bin/sh” or “#!/bin/bash” on the first line, then the system will treat all the commands in that script as different commands.

So I would suggest please start to add it.

Check below screenshots without #!/bin/bash:

I created sample running.bash file to explain the importance “#!/bin/sh” or “#!/bin/bash”.

Check below screenshots with #!/bin/bash:

We configured lots of cron job in our production Linux – PostgreSQL Server. Different schedules like monitor the database performance, monitor the backup process, take a database backup, replicate a database copy.

We set few monitor cron jobs at every 5 minutes. Now the situation is, few monitoring scripts are running even more than 5 minutes, and as per the scheduled, same script could start the execution after 5 minutes.

Now, the same script is running more than one time.

We should monitor this kind of situation, and we should prevent the next execution of the same script if it is already in running state. It also helps us to reduce an unnecessary load on the database server.

I prepared below two options; you can embed any one of it in your shell script.

Option script 1:

if pidof -x $(basename $0) > /dev/null; then
  for p in $(pidof -x $(basename $0)); do
    if [ $p -ne $$ ]; then
	echo "*****************************************************************"
	echo "  ***Warning*** Script $0 is already running...So existing...    "
	echo "*****************************************************************"
	exit 1
    fi
  done
fi

Option script 2:

isrunning=`ps -ef | grep $(basename $0) | grep -v $$ | grep -v grep | wc -l`
 
if [ "$isrunning" -ne 0 ]; then
   echo "**************************************************************"
   echo "  ***Warning*** Script $0 is already running... So exiting... "
   echo "**************************************************************"
   exit 1
fi

Recently, I shared one article on PostgreSQL schemas migration.

PostgreSQL: Script to copy Table Data from one Schema to another Schema

The migration activities are very common for all DBAs. I found lots of different questions on migration exercises.
In this post, I am sharing a Linux shell script to copy your table data from one PostgreSQL Server to another PostgreSQL Server using psql command line.

I am just migrating the data; please create a blank table at your destination/second database server.

This is a utility script. Further, you can modify the script for generic use something like by adding parameters for host_name, database_name, table_name and others

Linux Bash Shell Script for data migration between PostgreSQL Servers:

#!/bin/bash
psql \
    -X \
    -U user_name \
    -h host_name1 \
    -d database_name \
    -c "\\copy tbl_Students to stdout" \
| \
psql \
    -X \
    -U user_name \
    -h host_name2 \
    -d database_name \
    -c "\\copy tbl_Students from stdin"

You can also filter the column list something like below script:

#!/bin/bash
psql \
    -X \
    -U user_name \
    -h host_name1 \
    -d database_name \
    -c "\\copy (SELECT Rno, Studname FROM tbl_Students) to stdout" \
| \
psql \
    -X \
    -U user_name \
    -h host_name2 \
    -d database_name \
    -c "\\copy tbl_Students from stdin"

In the previous post, I shared shell script like store psql result into record array.
In this post, I am sharing one more shell script to store the result of psql SELECT query into a Variable of Bash Shell Script.

This handy script is useful for a beginner who just started to write a PostgreSQL DBA Bash Shell Script. You need this kind of demonstration to store the result of psql query into variable mostly like count of records.

The value of this variable we can use for further validation something like data exists or not in the Table.

In the below script, I used two variables to store the count of a table, but do not forget to notice option ‘-t’ in the second query which returns only tuple value and without option ‘-t’ it returns all formatted text as well like column name, returned a row count.

We need only tuple value so we must use ‘-t’ option. Please check the below demonstration.

#!/bin/bash

DB_NAME=$1
RecordCount1=$( psql -d ${DB_NAME} -c "select count(1) from tbl_students" )
RecordCount2=$( psql -d ${DB_NAME} -t -c "select count(1) from tbl_students" )

echo "RecordCount1:"$RecordCount1
echo "RecordCount2:"$RecordCount2

The result:

RecordCount1: count ------- 9 (1 row)
RecordCount2: 9

Now a day, I am developing different Linux shell scripts to monitor the PostgreSQL database server. Later, I will share those shell scripts.

But before that, I want to share few basic things which we can embed in any one of our Shell scripts.

In this post, I am sharing a small shell script to iterate the result of psql SELECT using RECORD ARRAY and WHILE LOOP.
Using that, we can do further processing and validation on each record.

Script to iterate the table result:

#!/bin/bash

DB_NAME=$1
psql -d ${DB_NAME} -At -c "select rno, studname, studclass from tbl_students" \
    | while read -a Record ; do
    rno=${Record[0]}
    studname=${Record[1]}
    studclass=${Record[2]}

    echo "${rno} ${studname} ${studclass}"
done

rno | studname
-----+----------+-----------
1 | Anvesh
2 | Neevan
3 | Toby
5 | Roy
4 | Jenny
6 | Kaviy
7 | Martin
8 | Laxmi
9 | Nion
(9 rows)