Useful Linux commands

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The one-stop shop for all your Linux command needs. Most commands listed will work on our machines, though that can vary based on installed packages and/or operating systems.

For almost every command, you can use the --help argument, <command> --help to get detailed information about the command as well as a list of its arguments.

File and Directory Management

ls

Lists all files and directories

  • ls -a - List all files, including hidden ones like .ssh, .bashrc, and so on.
  • ls -l - Include extra file information like permissions, owner, groups, modified date, and more.
  • ls -s - List size of files in blocks.
  • ls -S - List by size, largest first.

cd

Used to change the directory.

  • cd - When no path is specified, you will move to the user's home directory. As root, you will move to /root/. As your user, you will move to /eccs/home/<username>/.
  • cd /path/to/file - When changing directories, if you're not in the immediate parent directory, you need to specify the full path. For example, if my current working directory is /etc, I can cd jupyterhub since it's within /etc. However, if my current working directory is /bin, I would have to specify the full path for /etc/jupyterhub.
  • cd . and cd .. - The . and .. are used for adjacent movement. cd . "moves" you into the current directory, so basically no movement. cd .. moves you backwards one directory.
  • cd - - The dash specification takes you to the previous working directory regardless of adjacency.

pwd

Displays the current working directory. This can be usual on certain machines that don't display the full path of your current working directory, like Whedon. 

root@w0 plugins # pwd
/etc/nagios/plugins

In the above example, my current working directory is /etc/nagios/plugins, but the prompt only lists it at nagios.

mkdir

Used to create directories.

  • You can use mkdir directory_name to create it in your current working directory.
  • Additionally, you may specify the full path to create the directory somewhere else mkdir /path/to/directory_name.

rmdir

Used to remove empty directories.

  • You can ONLY remove empty directories. You cannot specify recursive or force.

mv

Used to move and rename files.

  • This command uses a mv /path/to/file /path/to/destination syntax. It follows the same rules as cd in that you need to specify the full path unless moving to the current working directory or an adjacent directory.
  • To rename a file, you can "move" it into a new one. mv old.txt new.txt will put the contents of old.txt into the newly created new.txt. This action will delete old.txt. NOTE: This WILL overwrite the file if you move it into a preexisting one.
  • mv -i - If you're worried about overwriting files, you can use the -i specification. This will prompt you for confirmation before overwriting.

rm

Used to remove files and/or directories. Unlike rmdir, you can use rm to remove directories. Even ones that aren't empty.

  • rm -d - Essentially the same as rmdir; it can only remove empty directories.
  • rm -r - Recursively removes files and directories from a directory, including itself.
  • rm -rf - The same process as rm -r, except it bypasses any locks or protections on files and deletes them. Only use this command if you are certain you understand what you're deleting, especially if you're root.
  • The * isis a powerful wildcard in Linux that allows you to match multiple files efficiently when using commands like rm. For example, take a look at this directory:
tdjones22@bowie:~/directory$ ls
123.txt  131.txt  14.txt  1file.txt  208.txt  263.txt  62.txt code.py code.rb config.conf

Let's say you want to remove every file that starts with the number 1. You can use rm *1 to remove any file matched with the first character being 1.
Files left after the command: 

208.txt  263.txt  62.txt code.py code.rb config.conf

Let's say you want to remove files with a specific extension. You can use rm *.txt to remove every file with the .txt extension.
Files left after the command: 

code.py code.rb config.conf

cp

Used to copy files and/or directories. Similar to rm in that you need to specify recursive when copying a non-empty directory. It follows a cp /path/to/file /path/to/destination syntax.

  • cp -r - Recursively copies a directory; the only way, using cp, to copy a directory that isn't empty.

find

A tool to find files using many criteria including name, type, contents of file, and more. A common syntax for this command is find /starting/directory -specification "name_of_file.txt".

  • find / -name "name.txt" - A typical usecase for find is searching by file name. If I've lost track of a file named found_file.txt, I can search the entire machine for it using this command:
tdjones22@bowie:~$ find / -name "found_file.txt" 2>/dev/null
/earlhamcs/eccs/users/tdjones22/directory/found_file.txt

This command will usually take some time to execute fully, so don't be alarmed when it hangs for a few seconds.


The starting directory was specified as / which is just the parent directory, meaning it searches the entire file system. The search type was specified as -name, which looks for matching named files. You can additionally add 2>/dev/null to silence all of the permission errors you'll get when searching all of the machine's directories.

  • find / -name "*.txt" -exec grep -iIl "contents to search for" {} + 2>/dev/null - If you know a portion of the contents of a file, but you don't know the name, you can use find in tandem with grep to find it. Though, it's far easier and more convenient to only use grep.

Let's say I have a missing text file, but I know it contains "solar power", I can find it like so: 

tdjones22@bowie:~$ find / -name "*.txt" -exec grep -RiIl "solar power" {} + 2>/dev/null
/eccs/home/tdjones22/test_find.txt

Like the previous command, this will take a hot minute to complete.


The starting directory was specified as / which is just the parent directory, meaning it searches the entire file system. We're looking for a file by its name specified as "*.txt", which looks for any files with the .txt extension. The -exec grep -iIl "solar power" {} + section is what searches through the file's contents. The grep arguments specify the following: -l - ignore case, -I - ignore binary files, and -l - display file name. You can additionally add 2>/dev/null to silence all of the permission errors you'll get when searching all of the machine's directories.

File Permissions and Ownership

chmod

Used to add, remove, or change file permissions. You normally specify permissions using an octal format, but you may also use symbolic as well. Here are some common chmod commands; full access for a user implies read, write, and execute ability:

  • chmod 600 | rw------- - The owner can read/write, but there is no access for others. Some use cases are private files and SSH keys.
  • chmod 644 | rw--w-r-- - The owner can read/write, but others can only read. Useful for config files and public web files.
  • chmod 660 | rw-rw---- - The owner and group can read/write, but there is no access for others. Useful for files used specifically in group collaboration.
  • chmod 664 | rw-rw-r-- - The owner and group can read/write, but others can only read. Useful for files used specifically in group collaboration.
  • chmod 700 | rwx------ - The owner has full access, but there is no access for others. Useful for private scripts (since you need the ability to execute a script) and sensitive directories.
  • chmod 755 - | rwxr-xr-x The owner has full access, but others can only read and execute. Useful for certain executables and web directories.
  • chmod 775 | rwxrwxr-x - The owner and group have full access, but others can only read and execute. Useful for group-managed directories.
  • chmod 777 | rwxrwxrwx - Everyone, and I mean everyone, has full access. So...don't use this unless absolutely necessary.

chown

Used to change which user, group, or both own files and directories.

  • chown user file - Change the owner of file to user.
  • chown user:group file - Change both owner and group of file.
  • chown :group file - Change only group of file.
  • chown -R user:group directory/ - Recursively change owner and group for all files in directory/.

chgrp

Used to change only the owner of both files and directories.

  • chgrp group file - Change the group of file to group.
  • chgrp group directory/ - Change the group of directory/.
  • chgrp group file1 file2 file3 - Change the group of multiple files.
  • chgrp -R group directory/ - Recursively change group for all files, including the directory itself, in directory/.

stat

Used to view various stats about files and directories such as size, blocks, type, inode, UID/GID, last access, creation date, and more. Let's see what happens when we run stat on the file test_find.txt: 

tdjones22@bowie:~$ stat test_find.txt
  File: test_find.txt
  Size: 27              Blocks: 8          IO Block: 4096   regular file
Device: fe04h/65028d    Inode: 25452759    Links: 1
Access: (0644/-rw-r--r--)  Uid: (11035/tdjones22)   Gid: (10000/sharedusers)
Access: 2025-03-28 13:42:16.638585831 -0400
Modify: 2025-03-28 11:31:47.781444104 -0400
Change: 2025-03-28 11:31:47.781444104 -0400
Birth: 2025-03-28 11:31:47.781444104 -0400

Let's talk about the output:

  • Size: 27 - The file is 27 bytes.
  • Blocks: 8 - The file occupies 8 blocks on the disk.
  • regular file - This is a "regular file", not a directory, symlink, etc.
  • Device: fe04h/65028d - The device ID of where the file is stored.
  • Inode: 25452759 - The inode number assigned to the file. Every file and directory has a unique inode number.
  • Links: 1 - The number of hard links to this file.
  • (0644/-rw-r--r--) - Permissions in octal and symbolic format.
  • Uid: (11035/tdjones22) Gid: (10000/sharedusers) - The user ID (Uid) of the owner and the group ID (Gid).
  • Access: 2025-03-28 13:42:16.638585831 -0400 - The last time the file was read.
  • Modify: 2025-03-28 11:31:47.781444104 -0400 - The last time the file's contents were modified.
  • Change: 2025-03-28 11:31:47.781444104 -0400 - The last time metadata (permissions, ownership, etc) was changed.
  • Birth: 2025-03-28 11:31:47.781444104 -0400 - When the file was first created.

Running this command on a directory displays the same format of output.

Process Management

ps

Used to display information about running processes. There are two common argument options when using ps: Unix options, which need to be preceded by a dash, and BSD options, which can be grouped and do not need a dash.

ps is a rather deep command with many specifications and shorthand syntax that is not necessarily intuitive. Here is the link to the official Bash documentation for PS1, which goes heavily into detail about the command with useful quick links at the top of the page.

Here are some common uses:

  • ps - Shows only the processes running in your current terminal session.

This is typical output for running ps alone: 

    PID TTY          TIME CMD
1980517 pts/0    00:00:00 bash
1980970 pts/0    00:00:00 ps
  • PID - The Process ID (PID) is the unique number that identifies a process.
  • TTY - TTY indicates the computer terminal that executed the process.
  • TIME - The total CPU time the process has used since it started.
  • CMD - The full command used to initiate the process. Includes paths and arguments.
  • ps -e/ps -A - Lists all of the running processes on the machine using the standard syntax.
  • ps -ef - Lists all of the running processes using the full format. Let's look at what it adds on top of the standard ps:
  • UID - The User ID (UID), usually a username, that owns the process.
  • PPID - The Parent Process ID (PPID).
  • C - The CPU usage in percentage over the process's runtime.
  • STIME - The time in HOUR:MINUTE format that the process started.
  • ps -u user - List all of the running processes for a specified user.
  • ps -eH - List all of the running processes in a hierarchical view. It visually indents processes in a human-readable syntax.
    920 ?        00:00:29   sshd
1980491 ?        00:00:00     sshd
1980516 ?        00:00:00       sshd
1980517 pts/0    00:00:00         bash
1983776 pts/0    00:00:00           ps

This is the process hierarchy of my bash shell (1980517), via ssh, running the ps -eH command.

  • ps aux - List all processes (a) for all users (u) using the wide syntax (x).
USER         PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
tdjones+ 1984316  0.0  0.0   9984  3488 pts/0    R+   14:17   0:00 ps aux

Let's talk about the wide syntax columns:

  • %CPU - CPU usage.
  • %MEM - Memory usage.
  • VSZ - Virtual memory size in KB. It includes all code, data, and shared libraries.
  • RSS - Resident Set Size (RSS) is the actual physical memory used in KB.
  • STAT - The state of the command.
  • ps -o - The -o argument allows you to specify the format of the output. Here is a command with all of the possible specifications:
ps -o user,pid,ppid,%cpu,%mem,vsz,rss,tty,stat,start,time,cmd

You can specify these in any order.


-- Major Overhaul: March 2025 by Tyler Jones [tdjones22] --

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