Files

cat — print a whole file

The cat command prints a file's contents to the terminal and joins several files into one stream.

mkfs — create a filesystem

The mkfs command creates an empty ext4 or xfs filesystem on a partition; the old data on it becomes unreachable.

/proc: processes and the kernel as files

/proc is a filesystem where the kernel shows processes (/proc/<pid>) and its own state: memory, load, parameters.

Basic file commands

Navigation and files — ls, cd, cp, mv, rm, cat, less, head, tail, terminal copy-paste.

Mounting and fstab

Mounting filesystems: the mount command, persistent mounts in /etc/fstab, UUIDs and options.

File permissions

The Unix permission model — read/write/execute, owner/group/other, special bits.

cd — changing directory

Moving around the filesystem: cd, cd -, cd ~, cd .., pushd/popd, CDPATH.

ls — list directory contents

The ls command lists files and directories with permissions, owner, size and modification time, hidden ones too.

mkdir — creating directories

Creating directories: -p (with parents), -m (permissions right away), brace expansion, rmdir.

mc — a terminal file manager

A two-panel file manager for the terminal: navigation, copying, an editor, SFTP.

stat — file metadata

Full file information — size, permissions, owner, timestamps (atime/mtime/ctime), inode.

Loop device — a file as a disk

A loop device shows a regular file to the system as a disk: you can partition, format and mount it.

file — determining file type

Figuring out what's inside a file (text, ELF, archive, image) by content, not by extension.

namei — permissions at every step of a path

The namei command walks a path part by part and shows the type, mode and owner of each directory down to the file.

cp — copy files and directories

The cp command copies files and directories, with overwrite prompts, preserved permissions and times, recursively.

mv — move or rename

The mv command renames a file or directory and moves it elsewhere; it silently replaces an existing target.

rm — remove files and directories

The rm command deletes files and directories at once, with no trash; -i and -I ask first, -r removes a tree.

ln — symbolic and hard links

Symlinks (ln -s) and hard links: how they differ, when to use which, common mistakes with relative paths.

test, [ ] and [[ ]] — checking conditions

test and [ ] check a file, a string or a number and answer with a code: 0 is yes, 1 is no; [[ ]] is the bash version.

grep — searching text

The grep command finds lines matching a pattern in files and command output: regex, recursion, context.

Disk space: df, du, inode

Where the space and inodes went — df, du, finding large files, pitfalls.

Paths — basename, dirname, realpath

Splitting a path into a name and a directory, getting an absolute path; a script's own directory.

Glob patterns and brace expansion

Shell substitutions — * ? [] {} — and how they differ from regular expressions.

find — searching for files

Finding files by name, size, time, permissions; acting on what's found.

lsof — open files and ports of processes

lsof shows which process holds a file, a port, or a deleted file that is still open.

Symbolic and hard links

The difference between a symlink and a hard link, when to use which, broken links.

chmod / chown — permissions and ownership

Changing permissions (chmod) and owner/group (chown) — numeric and symbolic syntax, umask, chgrp.

Filesystems: ext4, xfs, fsck

Filesystems: creating one with mkfs, labels and UUIDs, fsck checks, choosing between ext4 and xfs.

umask — the mask for new files and directories

umask is a process mask: which permissions to remove from a new file (from 666) and directory (from 777).

Special permission bits: setuid, setgid, sticky

Special permission bits: setuid and setgid run a program as its owner; setgid and sticky shape shared directories.

tar — archiving and extracting files

Packs files into a compressed archive; an archive with no top directory dumps its files into the current directory.

Extended permissions (ACL)

Flexible rights beyond rwx — getfacl/setfacl, per-user and per-group grants.

NFS network filesystem

Network directories over NFS — exporting on the server, mounting on the client, permissions.

rsync — file synchronization and copying

Copies only the differences between trees; --delete mirrors the source into the destination, removing anything extra.

xargs — building arguments from a stream

Turns stdin lines into a command's arguments; without -0, file names with spaces and newlines break the parsing.

qemu-img — virtual machine disk images

Creating and maintaining qcow2 disks — overlays, conversion, resizing.

restic — deduplicated backups

Incremental encrypted backups with snapshots; without the password the repository cannot be decrypted at all.

diff and patch — comparing and applying changes

Comparing files and applying patches: diff two configs, see what changed, apply a patch.

wget — downloading files over the network

Downloads files with resume and recursion; -r without -np and -l has no brakes and can mirror a whole site.

Disk quotas

Limiting space and file counts per user/group — enabling, configuring, reports.

git: change history, undo and a remote repository

A version control system: who changed what in the files, how to get back a working version and exchange changes.

GPG — encryption and signatures

GnuPG: GPG keys, encrypting files, signing and verifying, symmetric mode with a passphrase.

Dockerfile — an image recipe and docker build

A Dockerfile is an image recipe: base, files, build and start commands; docker build turns it into an image.

Samba — file sharing over SMB

An SMB server for Windows-compatible shares; the SMB password is separate from the system one, easy to forget to set.