Bash Scripting gist

basename find wc compgen -k compgen -b -z flag to redirect stderr: 2> to redirect stdoutput: > converting stderr to stdout: 2>&1 converting stdout to stderr: echo “hello” >&2 Add the nunber(File descriptor value) 1/2 depending on the deired output format after & for conversion. Silence standard ouput and error both: ./file.sh > /dev/null 2>&1

Heredoc Strings:

cat <<EOF
text
text
text
text
EOF

ssh -T ubuntu@ubuntumaschine <<EOF
ls 
EOF

Redirect output of heredoc string to a file.

cat <<EOF > helloworld.txt
text
text
text
text
EOF

ssh into remote and redirect the content of schema.sql into /var/log

ssh -T ubuntu@ubuntumachine bash "<<EOF >/var/log
cat /home/bob/docker_files/backup/sql_files/schema.sql                 
EOF"

Use heredoc string to quesry sql in docker

sudo docker exec my_postgres_container bash -c "psql -U postgres -d employees << EOF
select * from employee;
EOF"

Editing a file using file descriptor name: Assign file descriptor number to the file

exec 8<> /the/concerned/file.txt

Read the poistional value that needs to be edited.

cat /the/concerned/file.txt

a -> position 0
b -> position 1
c -> position 2

add character d at position 3

read -n 3 <& 8

address fo file descriptor to be sent at 3 position. Redirect position we opened above

echo "d" >& 8

Close the file desciptor number relationship

exec 8>&-

count number of the in the file.

cat sample.txt | grep -o "the" | wc -l

setting prevents errors in a pipeline from being masked

#!/bin/bash
set -o pipefail

If command produces a non-zero exit status, the script will immediately halt the execution and will exit with a status code 1, signifying an error.

|| exit 1

Error code with error message:

cat "${logfile}" | grep "ERROR" | sort  | uniq  -c || { echo "Error encountered in pipe commands" >&2; exit 1; }