Useful Commands

This is a non-exhaustive list of useful commands in Ubuntu. I include useful notes on their behavior that are often glossed over. I only list commands that I find are actually useful in practice, so I exclude commands like ls -a. This post already assumes you understand the basics of shell usage, like sudo.

Documentation

• cmd --help print help file
• cmd --help | less paginate help file
• man cmd read man page
• which cmd print file path of cmd; prints nothing for aliases

Running stuff

Each of these commands has subtle differences that should be understood.

• exec > file; ...; exit; redirect all stdout in ... into file
• exec file execute file replacing current shell process
• source script execute script in current shell
• bash script execute script in child shell process

Useful aliases and functions

• function hl { $1 --help | less; } read the help of the supplied command interactively
• alias la="ls -AF" I almost never use ls by itself because I usually WANT to see the dotfiles.
• alias ll="ls -lAF" If I’m using list format, I want to see everything.

Filesize Units

Because engineers hate each other, file sizes are often reported in powers of 1024 instead of powers of 1000, causing unnecessary confusion. That is, 1KB is often taken to mean 1 kibibyte (1024 bytes), 1MB sometimes means 1 mebibyte (1024^2 bytes), etc. The best part? The relative error is worse for larger file sizes, meaning the confusion caused by this is getting worse over time.

Luckily, many linux utilities give you the option of reporting file sizes in either system.

• du -h will report file sizes in human-readable units (powers of 1024).
• du --si will report file sizes in human-readable units (powers of 1000).
• df -h will report file sizes in human-readable units (powers of 1024).
• df -H will report file sizes in human-readable units (powers of 1000).
• ls -h will report file sizes

In general, engineers should prefer to be explicit but efficient, so if I were re-designing software from the ground up, I would use the following guidelines:

• By default, report data sizes in human-readable powers of 1000, using kB, MB, GB, etc.
• Use -b, or file_size_format="binary" to report file sizes in human-readable powers of 1024, using KiB, MiB, GiB, etc.
• Use --bytes or file_size_format="bytes" to report file sizes in bytes, with no grouping, for ease of parsing. This is not default because the default should be human-readability. This is because commands intended for machines only need to be written once, in a script file, whereas commands intended for humans are written often.

As it stands, one should probably implement -h, -H, and --si in command-line applications.

File Manipulation

• pwd Print Working Directory
• cd dir Change Directory
• ls List directory contents
  • ls -A almost all directory contents, excluding implied . and ..
  • ls -l list format, with more information
  • ls -F append type indicator to files
  • ls -l file show details of file
  • ls dir show contents of dir
  • ls -d dir list directory itself, instead of contents
  • ls -h human-readable file sizes in powers of two (K, M, G)
• tree helpfully display a tree of the current file structure
  • tree -d display only directories
  • tree -L 3 display a tree of three levels maximum
• exa is a really great replacement for ls and tree that has sensible defaults and some useful options
  • exa --group-directories-first -h group directories first, and show header for list view. I alias this as exa.
  • By default exa lists file sizes with human-readable si prefixes
    • -b for binary prefixes
    • -B for bytes
  • exa -DT show directory tree
  • exa -DTL3 show directory tree of depth 3
• mv src dest attempt to move src to dest/src if dest is a directory, otherwise rename src to dir, removing dir
  • mv file dir/. attempt to move src to dir/src, failing if dir is not a directory
• cp src dest attempt to copy src to dest/src if dest is a directory, otherwise overwrite dest with src
  • cp src dir/. attempt to copy src to dir/src, failing if dir is not a directory
• rm file remove file
  • rmdir empty_dir remove an empty directory
  • rm -r dir remove dir and all sub-directories; in general, don’t use -f

Permissions

Permissions are kinda weird. There are a lot of concepts to keep straight. If you ls -l file, you will see the following:

-rw-rw-r-- 1 owner group size modified file

The first dash represents the type of file it is. It may take on any of the following:

• - regular file
• d directory
• l symlink
• There are others

The next nine characters are the permissions of the file. They are split into groups of three. Each group can be rwx, or dashes. r-- means that group has only read access. r-x means that group has read and execute permissions. The first group of three is what the owner can do. The second group is what the group can do. The third group of three is what anyone other than the owner and group can do.

Permissions may also be specified in binary notation. 111 means the user has read, write, and execute permission. This is more commonly specified in octal as 7.

So 777 means the owner, group, and others can all read, write, and execute the file.

This is not a complete explanation of linux filesystem permissions.

A better explanation than mine can be had.

stat can also be used to see file permissions.

• chmod
• chage
• chown change ownership of file

Aliases

• alias list aliases and their definitions
  • alias alias show definition
  • alias alias="cmd arg arg" define alias
• unalias alias remove alias

Archives

tar [options] archive_name contents... compress file into file.tar.gz with gzip compression - f required if you want to specify file of archive - action - c create - x extract - optional compression - z with gzip compression - j with bzip2 compression - a automatically detect compression from file extension when extracting - common - cf create - czf create with gzip compression - xf extract - xzf extract with gzip decompression

System Information

• lshw -short will show you basic information about what hardware you are running. You should run it with sudo if you want the maximum amount of information.
• lspci to see all pci devices
• uname -m will show your instruction set
• ps -aux print information about running processes
• tty print file name of terminal connected to stdout
• lsusb list usb device info