How to find out if a package is installed in Linux?

1) How to find out if a package is installed or not in Linux

There are multiple ways to check find locate package is installed in linux machine or not

1.a) Using which command

The ‘which’ command returns an executable path that can be executed when the command is entered in the terminal.

# which vi
/usr/bin/vi

1.b) Using whereis command

The ‘whereis’ command is used to search the binary, source, and man page files for a given command.

# whereis vi
vi: /usr/bin/vi /usr/share/man/man1/vi.1p.gz /usr/share/man/man1/vi.1.gz

1.c) Using locate command

The ‘locate’ command performs faster than the find command because it uses an updatedb database, whereas the find command searches in real time.

# locate --basename '\nano'
/usr/bin/nano
/usr/share/nano
/usr/share/doc/nano

2) How to find out whether a package is installed or not in Linux, using package manager

2.a) On CentOS / Red Hat (RHEL) 6/7

Use yum command or rpm command to determine whether a given package is installed or not in Red Hat and their clones like CentOS, Oracle Linux.

Using yum command:

# yum list installed openssh

Using rpm command:

# rpm -qa nano
nano-2.3.1-10.el7.x86_64

Using dnf command:

# dnf list installed httpd