Table of Contents

If you’re new in Linux, these commands will help you to start learning the system, specially the command line.

Bash syntax

Check my post: Bash syntax.

Parameters

A command usually has this structure:

<command_name> <options> <file>

In this case, <options> and <file> are parameters. Command options have an option name and an optional (or required) option value. Option names usually start with a single dash (-) when is a one-letter, or two dashes (--) when the option name is a word. Some GNU commands (like tar) allow to specify one-letter option without typing the dash.

When you want to run a command where the <file> parameter starts with a dash, you need to tell to the command that <file> is not an option name. You can do this by typing -- before <file>. In the following example, we are going to create a file called -testfile:

touch -- -testfile
  • pwd: show the working directory.
  • cd <path>: change the working directory (cd files/). If you don’t add a <path>, it will change to the “home” directory for the user (run echo $HOME to see what is your home directory).
    • cd ..: change to the parent folder.
    • cd -: change to the previous working folder.
  • ls <path>: list files and folders. <path> is optional if you want to list files inside your working directory. Otherwise, you can use relative paths (relative to your working directory, like ./folder/, ../folder/) or absolute paths (like /home/user/).
    • You can use wildcards to filter the list: ls *.png.
    • ls -l: show files and folders permissions.
    • ls -a: include hidden files (files that start with .).
    • ls -lt: order by last modified time, ls -ltu: order by last accessed time.
    • ls --color=auto: colorizes the output.
  • du <path>: similar to ls, list files/directories and their sizes. Unlike ls, du shows folder size correctly (as a sum of the sizes of all their files).
    • du -h: use human-readable size format.
    • du --max-depth=<number>: limit the recursive listing to <number> levels.
    • du -a: include files in the list (not only directories).
    • du --apparent-size: print apparent sizes rather than device usage (useful for small files). You can use it with --block-size=1 for printing bytes instead of kilobytes.

Copy, move and delete content

  • cp <source> <target>: copy a file from a <source> to a <target>: For example, cp ./file.txt ./folder/. You can change the filename by typing the new filename in <target> (cp ./file.txt ./folder/file2.txt).
    • cp -r <folder> <target>: copy files and folders recursively. Check the difference when the target folder exists (copies the folder itself an its content) or does not exist (only copies the content).
    • cp --preserve=timestamps <source> <target>: preserve file timestamps. You can also preserve other attributes, by default --preserve preserves mode, ownership, timestamps.
  • mv <source> <target>: move a file or folder from <source> to <target>. It moves recursively.
  • rm <file>: remove a file.
    • rm -r <folder>: remove files and folders recursively.
    • rm -f <file>: remove a write-protected file (you need to have write permissions on the folder).

Create and delete folders

  • mkdir <folder>: create a folder.
    • mkdir -p <folder>: create parent folders if they don’t exist.
  • rmdir <folder>: remove an empty folder.

Processes

  • <command> &: run a command in the background (more info).
  • pkill <process name>: kill a process by its name.
  • kill <process PID or job number>: kill a process by its PID (Process ID) or job number (prepend ’%’ in this case) It sends SIGTERM signal.
    • kill -s <signal> <PID>: kill a process with a signal (type kill -l for a signals list). You can also type the signal name (SIGSTOP) or type signal number after a dash (-9).
  kill 1234
  kill %1
  kill -s 15 1234
  kill -s SIGTERM 1234
  kill -9 1234
  • killall <process name>: kill all instances of a process.
  • ps ax: list running processes owned by any user.
    • ps aux: similar to the above, with more info. Same as ps -ef.
    • ps -e -o pid,comm,%mem: customize fields.
    • ps --forest, ps f: display a process tree.
  • top: similar to ps ax, but updates automatically. Most people use htop (an enhanced version of top) for this task.
  • which <command name>: find where a command is being run from.
  • free: provides information about physical memory and swap space. By default, displays numbers in kilobytes. Add -m to show numbers in megabytes.
  • echo $0: shows the shell you are using (Bash, zsh, etc.). When using inside a script, displays script path (relative or absolute).
  • pgrep <process name>: find process ID.

Block devices and filesystems

  • lsblk: list available block devices.
    • lsblk -o <fields>: customize the output by selecting the columns.
  lsblk -o name,uuid,label
  • df: displays disk usage. Add -h to show disk space in a human-readable format.
  • du: shows the amount of space that is being used by files in a directory. Add -h to show disk usage in a human-readable format. Add -d <number> to specify a maximum depth.

Hardware information

  • lspci: list PCI devices. Add -v to get more info.
  • lsusb: list USB devices.
  • lscpu: info about CPU.

File info

  • file <file>: generic information about file type.
  • stat <file>: information about file permissions and date metadata (last accessed, last changed, etc.)
    • stat -c %U <file>: display only the file owner. You can also use %G for group and %A to show only file permissions.

File creation

  • touch <file>: creates an empty file if <file> does not exist.
    • touch -m -d "2022-01-01 01:00:00" urls.txt: change last modification time.
    • touch -a -d "2022-01-01 01:00:00" urls.txt: change last access time.
    • touch -d "2022-01-02 01:00:00" urls.txt : change modification and access time.
  • truncate <option> <file>: shrink or extend the size of a file.
    • truncate -s <size> <file>: <size> is an integer, with optional units: B,K,M,G,…
    • truncate -s 0 some_file: empty a file.
  • fallocate <option> <file>: similar to truncate.
    • fallocate -l <size> <file>
  • mktemp [<template>]: creates a temporary file or folder and prints their name. If <template> is defined, must include at least three ‘X’ at the end. Those ‘X’ will be replaced by random characters. The file will be created inside the working directory (if a relative path is used). If <template> is not defined, mktemp will create a file tmp.XXXXXXXXXX inside /tmp (or $TMPDIR if set).
    • mktemp -p <DIR> <template>: creates the file inside <DIR>. If <DIR> is not specified, uses $TMPDIR if set, else /tmp.
    • mktemp -d <template>: creates a temporary folder.

Users

Permissions

Network

  • wget <url>: download a file.
    • wget -O <filename> <url>: specify a name for downloaded file.
    • wget -P <path> <url>: specify an output directory.
    • wget -i <list file>: specify a file with the URLs (one per line).
    • wget -c <url>: resume a stopped download (if server is compatible).
  • ip route or ip r: display route table (shows local IP).

Keyboard shortcuts

  • Ctrl + L or clear: clear the terminal.
  • Ctrl + +: increase the terminal font size.
  • Ctrl + -: decrease the terminal font size.
  • Ctrl + R: Search through command history. Type a search and press Enter to execute the found command, Ctrl + R to go to next match, or press the right arrow to edit the command. Ctrl + G to quit search.
  • Ctrl + A: move to the beginning of the line.
  • Ctrl + E: move to the end of the line.
  • Ctrl + K: remove text from the current cursor position to the end of the line.
  • Ctrl + U: remove text from the current cursor position to the beginning of the line.
  • Ctrl + C: stop the current job.
  • Ctrl + Z: suspend the current job.
  • Alt + .: insert the last argument of the previous command.
  • !!: repeat the previous command.
$ pacman -Sy
error: you cannot perform this operation unless you are root.
$ sudo !!
sudo pacman -Sy
[sudo] password for ricardo:

More

  • head <file>: show the first 10 lines of a file. You can specify any number of lines with -n <number>.
    • head -n -3: show all except the last 3 lines.
  • tail <file>: show the last 10 lines of a file. You can specify any number of lines with -n <number>.
    • tail -n +3: show all except the first three lines.
    • tail -f: output appended data as the file grows, useful for viewing log files.
  • read -p "<prompt>" <variable>: read from user input and assign to a variable.
$ read -p "Confirm (y/n): " res; echo $res 
Confirm (y/n): y
y
  • read -n 1 <variable>: accepts only one character for input.
  • man <page>: show a reference manual about a topic (more info).
    • man <section number> <page>: show an specific section (e.g.: you can access crontab(5) by typing man 5 crontab)
  • date: show date and time.
    • date +FORMAT: shows date/time with the specified format: For example, date +%s shows seconds since the Epoch.
    • date -d <date>: display <date> (date -d 20210101, date -d "2 weeks ago").
  • run-parts <folder>: execute all scripts inside a folder, sequentially.
  • wc [options] <file>: count words, lines, characters of a file.
    • wc -l <file>: count lines. It also displays the filename, to show only the number of lines, run: wc -l < <file>.
  • watch <command>: execute a program periodically, showing its output fullscreen. By default, the command is run every 2 seconds.
    • watch -n <seconds> <command>: specify the interval.
    • watch -t <command>: turn off the header.
    • watch '<command> | <command>': you can use pipes.
  • lsof: list open files.
    • lsof <directory>, lsof -p <PID>, lsof -u <users>
    • lsof -i [46][protocol][@hostname|hostaddr][:service|port]: e.g.: lsof -i TCP:https
    • +L1: list open files with links fewer than 1, i.e. removed/unlinked files.
    • +D <directory>: search for any open instances of <directory> and all files and directories it contains.
  • ldconfig -p | grep <lib name>: find out if <lib name> library is installed.
Test with this online terminal:

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