Unix Primer for Ubuntu

Basic commands

Files

List files in a directory

ls path/

List all files in a directory with permissions and include hidden files

ls -la path/

Change into a directory

cd path/

View an entire file in the console

cat file.txt

View the first 100 lines of a file

head -n 100 file.txt

View the last 100 lines of a file

tail -n 100 file.txt

Launch nano to create/edit a file

nano file.txt

Use Ctrl+X to exit nano.

Create a directory

mkdir path/

Remove a file

rm file

Remove a directory and its contents

rm -r path/

Create a symbolic link to a file

sudo ln -s <file-name> <link-name>

Compress and archive one or more files (tar.gz)

tar -czvf archive.tar.gz /path

Decompress and extract an archive to a specified directory

tar -xzvf archive.tar.gz -C /path

For this command, you can use the --strip-components=n flag to strip n subdirectory paths from the files in the original archive.

File permissions

Viewing and basic concepts

To see the current owner and permissions for all files in a directory, use the ls -l command.

$ ls -l /var/repo/userfrosting.git

total 44
-rwxrwxr-x 1 alex alex  478 Feb 16 02:04 applypatch-msg.sample
-rwxrwxr-x 1 alex alex  896 Feb 16 02:04 commit-msg.sample
-rwxr-xr-- 1 alex alex  138 Feb 22 02:18 post-receive
-rwxrwxr-x 1 alex alex  189 Feb 16 02:04 post-update.sample
-rwxrwxr-x 1 alex alex  424 Feb 16 02:04 pre-applypatch.sample
-rwxrwxr-x 1 alex alex 1642 Feb 16 02:04 pre-commit.sample
-rwxrwxr-x 1 alex alex 1239 Feb 16 02:04 prepare-commit-msg.sample
-rwxrwxr-x 1 alex alex 1348 Feb 16 02:04 pre-push.sample
-rwxrwxr-x 1 alex alex 4898 Feb 16 02:04 pre-rebase.sample
-rwxrwxr-x 1 alex alex 3610 Feb 16 02:04 update.sample

The first part of each line contains a -, followed by three sets of three characters each. The first set of three tells you the owning user's permissions, the second set tells you the owning group's permissions, and the last set tells you the permissions for "other" users. "Other" users means any other Ubuntu user account on your server.

The next pieces of significant information are the name of the owning user and owning group. This is important, because Unix permissions are always defined relative to the owning user and owning group.

For example, let's look at this entry:

-rwxr-xr-- 1 alex alex  138 Feb 22 02:18 post-receive

We can see that the owning user is alex, and the owning group is also alex (Whenever you create a user in Linux, it automatically creates a group of the same name).

Then we have the following permissions:

• User: rwx. The owning user (alex) has full read, write, and execute permissions for this file.
• Group: r-x. The owning group (alex) has read and execute permissions for this file.
• Other: r--. Other users only have read permissions for this file.

Changing the owning user/group for a file

sudo chown <user>:<group> file.txt

Changing permissions

sudo chmod u+r,g+r,o-w file.txt

The arguments u, g, and o refer to the owning user, owning group, and other users, respectively. The + symbol means that we are adding permissions (use - instead to remove permissions). The symbols afterwards are the permissions we are adding/removing. They can be any combination of r (read), w (write), and x (execute).

Set default permissions

When new files are created in Unix, they get their permissions primarily from the file mode creation mask, also known as the umask.

If we want to change how permissions are set on newly created files in a directory (for example, when pushed there by git), we can use setfacl:

sudo setfacl -d -m g::rwx /my/path

The -d indicates default permissions, and -m g::rwx says to grant read, write, and execute permissions for the owning group (g) on any new files created in the directory.

Just as with chmod, you can use u for the owning user, or o for "all other users".

Package management

Install a package

apt-get install <package-name>

Update a package

apt-get upgrade <package-name>

Remove a package

apt-get remove <package-name>

Processes

Search for a process

ps aux | grep "<description>"

This will display a list of matching processes, along with their process id (pid).

Kill a process by pid

First try kill <pid>. If that doesn't work, try kill -9 <pid>.

Default locations in Ubuntu

User accounts

• root: The root user. Has the highest level of privileges.
• www-data: The default account under which nginx and apache run.
• backup: The "backup" user account.

Automation With Cron

TODO