Everyday Notes from a Linux Administrator
CentOS
The sed utility can be used to print the contents of a
file, substitute a line (or multiple lines), and then save the file. In
contrast to grep, sed can substitute a line or multiple lines in a file and perform an in-place update of that file.
The simplest sed invocation when substituting foo for bar is:
$ sed 's/foo/bar/' inputfileThere are several different authentication schemes that can be used on Linux systems. The most commonly used and standard scheme is to perform authentication against the /etc/passwd and /etc/shadow files.
/etc/passwd is a plain text-based database that contains information for all user accounts on the system. It is owned by root and has 644 permissions. The file can only be modified by root or users with sudo privileges and readable by all system users.
Modifying the /etc/passwd file by hand should be avoided unless you know what you are doing. Always use a command that is designed for the purpose. For example, to modify a user account, use the usermod command, and to add a new user account use the useradd command.
Every Linux command has one or more options and flags to perform different operations. One of the useful and important option allow us to simulate Linux commands but do not actually change the system. For instance, we can simulate the process of installation or removal of a package or program, but without actually installing or removing the intended package from a Linux system.
Continue reading “How To Simulate Linux Commands”There are several different authentication schemes that can be used on Linux systems. The most commonly used and standard scheme is to perform authentication against the /etc/passwd and /etc/shadow files.
/etc/shadow is a text file that contains information about the system’s users’ passwords. It is owned by user root and group shadow, and has 640 permissions.
Missing rc.local for adding commands to run on startup? Here’s how to set up similar functionality with today’s systemd.
The rc.local file was—and in some cases still is—the place for Linux sysadmins to put commands that need to be run at startup. Use of the rc.local
file is not only deprecated but after a couple of hours worth of
attempts, was not working in any event. This despite the fact that the systemd documentation mentions the use of a “generator” that generates systemd services from an rc.local file if one exists.
If an open file is deleted accidentally, it is possible to use lsof to recreate a copy of the file; provided this is done before the file is closed by the application holding it open.
If you have inadvertently removed a file from the filesystem it is still recoverable if the application using the file it still running. This is because the inode is still open and therefore the data blocks are still on the disk until the application closes the file or exits.
Continue reading “Using lsof to Recover Deleted Files”Docker is the most popular container technology. It is designed to make it easier to create, deploy, and run applications by using containers. Docker behaves similar to a virtual machine hypervisor. But unlike a virtual machine, rather than creating a whole virtual operating system, Docker allows applications to use the same Linux kernel as the system that they’re running on and only requires applications be shipped with things not already running on the host computer. This gives a significant performance boost and reduces the size of the application.
Continue reading “Essential Docker commands”You may find Ad-hoc mode (from Part II) easy to use on day-to-day tasks such as for quick systems checks or when updating a config file on multiple systems. Stepping up simple ad-hoc commands are Ansible playbooks (from Part III) and Ansible roles which are very powerful ways to utilize Ansible’s features.
Continue reading “Ansible Part IV: Roles Overview”