Shell-script-part-1

What is sh?

sh (or the Shell Command Language) is a programming language described by the POSIX standard. It has many implementations (ksh88, Dash, ...). Bash can also be considered an implementation of sh (see below).

Because sh is a specification, not an implementation, /bin/sh is a symlink (or a hard link) to an actual implementation on most POSIX systems.

What is Bash?

Bash started as an sh-compatible implementation (although it predates the POSIX standard by a few years), but as time passed it has acquired many extensions. Many of these extensions may change the behavior of valid POSIX shell scripts, so by itself Bash is not a valid POSIX shell. Rather, it is a dialect of the POSIX shell language.

Bash supports a --posix switch, which makes it more POSIX-compliant. It also tries to mimic POSIX if invoked as sh.

sh = bash?

For a long time, /bin/sh used to point to /bin/bash on most GNU/Linux systems. As a result, it had almost become safe to ignore the difference between the two. But that started to change recently.

Some popular examples of systems where /bin/sh does not point to /bin/bash (and on some of which /bin/bash may not even exist) are:

1. Modern Debian and Ubuntu systems, which symlink sh to dash by default;
2. Busybox, which is usually run during the Linux system boot time as part of initramfs. It uses the ash shell implementation.
3. BSD systems, and in general any non-Linux systems. OpenBSD uses pdksh, a descendant of the KornShell. FreeBSD's sh is a descendant of the original Unix Bourne shell. Solaris has its own sh which for a long time was not POSIX-compliant; a free implementation is available from the Heirloom project.

 Determine the shell type:

----------------------------------Monitor the linux node health ------------------

Sample script:

#!/bin/bash

echo "-----------------------"
echo "System Health Snapshot"
echo "-----------------------"

# Check CPU Usage
echo -e "\n1. CPU Usage:"
top -b -n 1 | grep '%Cpu'

# Check Memory Usage
echo -e "\n2. Memory Usage:"
free -m

# Check Disk Space
echo -e "\n3. Disk Space:"
df -h

# Check Network Activity
echo -e "\n4. Network Activity:"
iftop -t -s 2

# Check Running Processes
echo -e "\n5. Running Processes:"
ps aux

# Check System Uptime
echo -e "\n6. System Uptime:"
uptime

# Check System Load Average
echo -e "\n7. System Load Average:"
w

# Check Kernel Messages
echo -e "\n8. Kernel Messages:"
dmesg | tail -n 5

# Check System Logs
echo -e "\n9. System Logs (last 10 lines):"
tail -n 10 /var/log/syslog

# Check Users Currently Logged In
echo -e "\n10. Users Currently Logged In:"
who

# Check CPU/RAM count like 2gb or 4gb

nproc
echo "-----------------------"