Linux Commands and Tools

What it is

Linux is an open source operating system (OS) used on servers, desktops and many devices. Technicians meet it mostly through the command line, called the shell or terminal. The common commands are the same across most distributions (distros), the packaged versions of Linux.

How it works

Files and navigation

pwd                      # print the current directory
ls -l                    # list files with permissions and owners
cd /etc                  # change directory
cp -r docs docs_backup   # copy a folder and its contents
mv old.txt new.txt       # move or rename
rm old.txt               # delete a file
rm -r olddir             # delete a folder and its contents
cat notes.txt            # show a file
nano notes.txt           # edit a file in a simple editor (vi is another common editor)
man ls                   # open the manual page for a command

Linux is case sensitive, so File.txt and file.txt are different files. Deleted files do not go to a recycle bin when rm is used.

Searching

grep "error" syslog          # lines that contain the word
find /home -name "report.txt"  # locate a file by name

grep searches inside files for text. find searches the file system for files by name or other attributes.

Permissions and ownership

ls -l shows ten characters for each item, such as -rwxr-xr--. The first is the type, then three sets of read (r), write (w) and execute (x) for the owner, the group and everyone else.

Add the numbers for each set. So 7 is read, write and execute, 6 is read and write, and 5 is read and execute.

chmod 755 script.sh      # owner rwx, group r-x, others r-x
chmod 644 notes.txt      # owner rw-, group r--, others r--
chown alex:staff notes.txt   # change owner and group

chmod changes permissions and chown changes the owner. A script needs the execute permission before it can be run directly by name.

Privileges

The root user is the administrator account. Rather than log in as root, a user runs a single command with elevated rights using sudo (superuser do). The su command switches to another user account.

sudo apt update
su - alex

Processes, disks and network

The older ifconfig command also shows interface settings and may still appear on older systems.

Filesystem management

sudo fsck /dev/sdb1          # check and repair a file system
sudo mount /dev/sdb1 /mnt    # attach the device at /mnt
sudo umount /mnt             # detach it again

fsck checks and repairs a file system, and it should be run on an unmounted volume. mount attaches a device to a folder so its files can be used, and umount detaches it. Note the spelling umount, with no n.

More disk and network commands

du -sh /var/log                       # size of one folder
curl -I https://example.com          # fetch only the headers
curl -O https://example.com/a.zip    # download a file
traceroute example.com               # list the hops to a host

df reports space for each mounted file system, while du reports how much space a folder uses. curl transfers data to or from a URL. traceroute is the Linux counterpart of tracert.

Installing software

Package managers fetch and install software from repositories.

sudo apt-get install nano
sudo dnf install nano

Common configuration files

OS components

Use systemctl to control services, for example sudo systemctl start cups, sudo systemctl stop cups and systemctl status cups.

Other features

A graphical desktop environment sits on top of the OS for users who prefer it. Linux file systems commonly use ext4.

Samba lets a Linux computer share files and printers with Windows using Server Message Block (SMB). Backups can use tools such as rsync and tar, and cron can run them on a schedule.