Hi Juliett,
if you try to install ansible with "yum" , you'll have only ansible (2.6, 2.7, 2.8 and 2.9 release available):
(and not 2.5.7 release)
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We will install ansible 2.5.7 with an another method:
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An ansible repository with all release of ansible for Redhat/CentOS/Fedora:
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(i.e https://releases.ansible.com/ansible/rpm/release/epel-7-x86_64/)
install wget:
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yum install wget
download ansible 2.5.7 release:
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wget https://releases.ansible.com/ansible/rpm/release/epel-7-x86_64/ansible-2.5.7-1.el7.ans.noarch.rpm
install ansible 2.5.7 :
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yum localinstall ansible-2.5.7-1.el7.ans.noarch.rpm
check ansible's version:
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ansible --version
ansible 2.5.7
Good Luck !