Linux

67 commands across 19 tools

`chmod`

Give everyone read access to a directory tree
Caution
```
chmod -R a+rX path
```
Recursively grants read to all users, plus execute on directories and on files that are already executable.
- permissions
- files
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Make a file executable
```
chmod +x file
```
Adds execute permission so the file can be run as a program or script.
- permissions
- files
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Set permissions with an octal mode
```
chmod 644 file
```
Replaces the permissions of the file with the octal mode, such as 644 for owner write and read for everyone.
- permissions
- files
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`chown`

Change the owner of a directory tree
Caution
```
sudo chown -R user:group path
```
Recursively sets the owning user and group of the directory and of everything inside it.
- permissions
- files
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`curl`

Download a file
```
curl -fLO https://example.com/file.tar.gz
```
Saves the file in the current directory under its name from the URL, following redirects and failing on HTTP errors.
- http
- download
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Fetch a URL
```
curl https://example.com
```
Sends a GET request and prints the response body to the terminal.
- http
- network
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Print only the HTTP status code
```
curl -s -o /dev/null -w '%{http_code}\n' https://example.com
```
Requests the URL and prints just the HTTP status code it returns, discarding the body.
- http
- troubleshooting
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Send a JSON POST request
```
curl -H 'Content-Type: application/json' -d 'json' https://example.com/api
```
Sends the JSON body in a POST request with the matching Content-Type header.
- http
- api
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Send a POST request with form data
```
curl -d 'name=bob' https://example.com/form
```
Posts URL-encoded form fields to the URL, as an HTML form would.
- http
- api
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Show response headers, following redirects
```
curl -sSL -D - -o /dev/null https://example.com
```
Follows redirects and prints the response headers of each hop while discarding the body.
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`df`

Show free disk space
```
df -h
```
Lists every mounted filesystem with its size, used and available space in human-readable units.
- disk
- filesystem
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Show inode usage
```
df -i
```
Shows used and free inodes per filesystem, which explains "no space left" errors on a disk that still has free bytes.
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Show which filesystem holds a path
```
df -h .
```
Shows the filesystem that contains the file or directory, with its size and free space.
- disk
- filesystem
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`du`

List items above a size threshold
```
du -ah -t 1G .
```
Lists files and directories under the path whose size is at least the threshold.
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List the biggest items in a directory
```
du -ah . | sort -rh | head -n 20
```
Shows the largest files and directories under the path, biggest first.
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Show the size of each subdirectory
```
du -h --max-depth=1 . | sort -h
```
Prints the disk usage of every direct subdirectory in human-readable units, sorted from smallest to largest.
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Show the total size of a directory
```
du -sh .
```
Prints a single human-readable total for the directory and everything inside it.
- disk
- usage
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`find`

Delete empty files and directories
Destructive
```
find path -empty -delete -print
```
Deletes every empty file and empty directory under the path and prints each one it removes.
- files
- cleanup
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Find directories by name, ignoring case
```
find . -type d -iname '*cache*'
```
Recursively lists directories whose name matches the pattern, regardless of upper or lower case.
- files
- search
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Find files by extension
```
find . -type f -name '*.log'
```
Recursively lists regular files whose name ends with the given extension under a directory.
- files
- search
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Find files larger than a given size
```
find / -type f -size +1G 2>/dev/null
```
Recursively lists regular files bigger than the given size under a directory, hiding permission errors.
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Find files modified in the last days
```
find . -type f -mtime -1
```
Recursively lists regular files whose content changed within the given number of days.
- files
- search
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Find files while excluding a path
```
find . -name '*.py' -not -path '*/node_modules/*'
```
Lists files matching a name pattern but skips everything whose path matches the excluded pattern.
- files
- search
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Find files within a size range
```
find . -type f -size +500k -size -10M
```
Recursively lists regular files bigger than the minimum and smaller than the maximum size.
- files
- disk
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`free`

Show memory usage
```
free -h
```
Displays total, used and available RAM and swap in human-readable units.
- memory
- monitoring
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`grep`

Exclude lines matching a pattern
```
grep -v "pattern" file
```
Prints only the lines of the file that do not match the pattern.
- search
- text
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Print only the matched text
```
grep -o "pattern" file
```
Outputs just the part of each line that matches, one match per line, instead of the whole line.
- search
- text
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Search for a literal string
```
grep -F "text" file
```
Prints lines containing the exact text, treating regex characters such as dots and brackets literally.
- search
- text
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Search for a pattern in a file
```
grep -n "pattern" file
```
Prints every line of the file that matches the regular expression, prefixed with its line number.
- search
- text
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Search recursively in a directory
```
grep -rnI "pattern" .
```
Searches every file under the directory, skipping binary files, and prints matches with file name and line number.
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Show lines around each match
```
grep -C 3 "pattern" file
```
Prints each matching line together with the given number of lines before and after it.
- search
- text
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`ip`

Show IP addresses
```
ip -br addr
```
Lists each network interface on one line with its state and IPv4 and IPv6 addresses.
- network
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`journalctl`

Delete old journal logs
Caution
```
sudo journalctl --vacuum-time=2d
```
Removes archived journal files older than the given age to free disk space.
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Follow the logs of a service
```
journalctl -u service -f
```
Shows the latest log entries of the service and keeps streaming new ones as they are written.
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Follow the system log
```
journalctl -n 100 -f
```
Prints the most recent log lines and keeps streaming new entries as they arrive.
- logs
- systemd
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Show errors from the previous boot
```
journalctl -b -1 -p err
```
Lists messages of priority error or worse logged during the boot before the current one.
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Show logs within a time range
```
journalctl --since "1 hour ago" --until "now"
```
Prints log entries written between the two times, which accept dates or phrases like yesterday or 1 hour ago.
- logs
- systemd
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`lsof`

Find which process uses a port
```
sudo lsof -i :8080
```
Lists the processes that have the network port open, with their PID, user and connection state.
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`pgrep`

Find processes by name
```
pgrep -af pattern
```
Lists the PID and full command line of every process whose command line matches the pattern.
- processes
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`rsync`

Copy a directory preserving permissions and times
```
rsync -a source destination
```
Recursively copies files while keeping permissions, ownership, timestamps and symlinks as they are.
- sync
- files
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Copy a directory's contents, not the directory
```
rsync -a source/ destination
```
Copies everything inside the source directory into the destination, without creating a source subfolder there.
- sync
- files
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Copy to a remote host over SSH on a custom port
```
rsync -a -e 'ssh -p 2222' source user@host:destination
```
Transfers files to a remote machine through an SSH server listening on a non-default port.
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Copy with compression and progress
```
rsync -azhP source destination
```
Transfers files compressed over the network, shows progress and keeps partial files so an interrupted copy can resume.
- sync
- transfer
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Mirror a directory, deleting extra files
Destructive
```
rsync -a --delete source/ destination
```
Makes the destination an exact copy of the source, deleting destination files that no longer exist in the source.
- sync
- backup
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Preview what rsync would copy
```
rsync -av --dry-run source destination
```
Lists the files that would be transferred without copying or changing anything.
- sync
- files
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`ss`

Check if a port is listening
```
ss -ltn src :8080
```
Shows the TCP socket listening on the local port, or nothing if no process listens there.
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List listening ports with their process
```
sudo ss -tulpn
```
Shows every TCP and UDP socket waiting for connections, with numeric ports and the process that owns it.
- network
- ports
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Show connections on a local port
```
ss -tn src :443
```
Lists TCP connections whose local side uses the port, with numeric addresses.
- network
- ports
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`ssh`

Connect through a jump host
```
ssh -J user@jump_host user@host
```
Reaches a server that is only accessible from a bastion by hopping through the bastion first.
- remote
- bastion
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Connect to a remote server
```
ssh user@host
```
Opens an interactive shell on the remote machine as the given user.
- remote
- network
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Connect with a specific private key
```
ssh -i ~/.ssh/id_ed25519 user@host
```
Opens a session using the given private key file instead of the default ones.
- remote
- keys
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Forward a local port to a remote service
```
ssh -L 8080:localhost:80 -N user@host
```
Makes a service reachable from the server available on a local port, through the SSH tunnel.
- remote
- tunnel
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Open a SOCKS proxy through a server
```
ssh -D 1080 -N user@host
```
Starts a local SOCKS proxy on the port that routes traffic through the remote server, without opening a shell.
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`ssh-copy-id`

Install your public key on a server
```
ssh-copy-id -i ~/.ssh/id_ed25519.pub user@host
```
Appends your public key to the remote authorized_keys file so later logins use the key instead of a password.
- remote
- keys
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`systemctl`

Check the status of a service
```
systemctl status service
```
Shows whether the service is running, its main PID, memory use and its most recent log lines.
- systemd
- services
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Enable a service at boot and start it
Caution
```
sudo systemctl enable --now service
```
Configures the service to start automatically at boot and starts it right away.
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List failed units
```
systemctl --failed
```
Shows every systemd unit whose last start or run ended in an error.
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List running services
```
systemctl list-units --type=service --state=running
```
Lists only the service units that are currently active and running.
- systemd
- services
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Reload systemd unit files
```
sudo systemctl daemon-reload
```
Makes systemd reread unit files after you create or edit one, without restarting any service.
- systemd
- services
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Restart a systemd service
Caution
```
sudo systemctl restart service
```
Stops the service and starts it again, briefly interrupting it for its clients.
- systemd
- services
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Show the unit file of a service
```
systemctl cat service
```
Prints the unit file and every drop-in override that applies to it, each preceded by its path.
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`tail`

Follow a log file
```
tail -f /var/log/syslog
```
Prints the last lines of the file and keeps showing new lines as they are appended.
- logs
- files
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`tar`

Archive a directory's contents with relative paths
```
tar -czf archive.tar.gz -C path .
```
Compresses everything inside the directory so that paths in the archive start at its root instead of the full path.
- archive
- compression
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Create a gzipped archive
```
tar -czf archive.tar.gz paths
```
Packs the given files and directories into a single gzip-compressed tar archive.
- archive
- compression
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Extract an archive in the current directory
```
tar -xvf archive
```
Unpacks a tar archive, compressed or not, into the current directory and lists each extracted file.
- archive
- compression
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Extract an archive into a directory
```
tar -xf archive -C path
```
Unpacks the archive into the target directory, which must already exist.
- archive
- compression
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List the contents of an archive
```
tar -tvf archive
```
Shows every file in the archive with its permissions, size and date, without extracting anything.
- archive
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