Linux Sysadmin

alternatives command (e.g. to set to a specific default Java version)
- sudo alternatives --config java
- Introduction to the alternatives command in Linux
- To remove an alternatives entry: asroot alternatives --remove java /home1/hub/java11_64.ansible/jdk-11.0.20.1+1/bin/java
User accounts:
- Add user to sudoers
  One method is to add the user to wheel group: # usermod -aG wheel username. Now log in again as username and run groups command.
- Create group: sudo groupadd ansible
- Create user (also add user to sudoers via wheel group membership):
  sudo useradd -g ansible -G wheel ansible
- Set password: passwd ansible
- Remove prompt for password when using sudo (not most secure, but very handy for using ansible etc):
  Run visudo
  Comment-out and comment these lines, to leave them like this:
```
## Allows people in group wheel to run all commands
#%wheel ALL=(ALL)       ALL
## Same thing without a password
%wheel  ALL=(ALL)       NOPASSWD: ALL
```
  To test, running sudo id should return root
Cron
ps
- Seeing all of the command-line: ps auxww | grep java
System-wide LD_LIBRARY_PATH update:
- LD_LIBRARY_PATH - how to update for a system service: Add the directory to /etc/ld.so.conf or a new file in /etc/ld.so.conf.d/, depending on distro. After that, you must run (at least on Redhat) ldconfig as root.
- See also https://developer.ibm.com/technologies/linux/tutorials/l-lpic1-102-3/.
System-wide PATH update:
- See files in /etc/profile.d
rpm:
- List installed packages: rpm -qa
- List files in an uninstalled rpm: rpm -qpl ./BaseOS/Packages/yum-utils-4.0.8-3.el8.noarch.rpm
- List files in an installed rpm: rpm -ql postgresql12-server.x86_64 | grep conf
- Idenitfy the package a file came from: rpm -qf /usr/bin/svn
- Install an rpm, and automatically install any dependencies (via yum): yum --nogpgcheck localinstall VirtualBox-6.1-6.1.12_139181_el8-1.x86_64.rpm
systemd:
- Debug service startup faiure: journalctl -xe -u redis
- List all services: systemctl list-unit-files | grep -i postgres or sudo systemctl list-units --type=service
- Example config file path: /usr/lib/systemd/system/MFSafeNet.service
top:
- top -c and 200% cpu! (type shift+i to see shared per-cpu core usage): See here
- Filter top output by process name: asroot top -c -p $(pgrep -d',' -f cascd) (see here
yum:
- Yum cheatsheet
- List files in an uninstalled package:
  yum repoquery --list postgresql12-contrib.x86_64
- List all available versions of each package, rather than the most recent version:
  yum --showduplicates --disablerepo="*" --enablerepo="pgdg10" list available
- Install a package version that is not the most recent:
  yum --showduplicates --disablerepo="*" --enablerepo="pgdg10" install postgresql10-server-10.12
  Another example:
  yum --showduplicates --disablerepo="*" --enablerepo="pgdg10" install postgresql10-odbc-10.03.0000-1PGDG.rhel7
  Note: Take the package base name (e.g. “postgresql10-server”) and then add the version (e.g. “10.12”). See also here.

Written on October 13, 2020