# Unix Command Line for the Molecular Ecologist

Basic familiarity in the Unix command line opens a whole world of bioinformatics tools and analysis opportunities. This guide introduces the most important commands and concepts you’ll need to get started.

## Quick cheat sheet

• Change directories: cd [PATH]
• List files in directory: ls (current directory) or ls [PATH] (another directory)
• See file sizes in a directory: ls -lh [PATH]
• Print current directory: pwd
• Make a directory: mkdir [PATH]
• Remove an empty directory: rmdir [PATH]

### Working with files

Basic operations:

• Move/rename a file: mv [FILE] [NEW PATH]
• Delete a file (permanently!): rm [FILE]
• Delete an entire directory (permanently!): rm -r [DIRECTORY]
• Copy a file: cp [FILE] [NEW PATH]
• Copy an entire directory: cp -r [DIRECTORY] [NEW PATH]

Useful tricks:

• Download a file: wget -O [OUTPUT FILE] [URL] or curl -o [OUTPUT FILE] [URL] (wget may not be available in OS X)
• View a file: head -[NUMBER OF LINES] [FILE] or less [FILE]
• Create a blank file: touch [FILE]
• Edit a file: nano [FILE]
• Search within a file: grep [QUERY] [FILE] or grep [QUERY] [FILE] -A [AFTER] -B [BEFORE]
• Create a symbolic link to a file: ln -s [FILE] [NEW LINK]
• Wildcards: * matches any number of characters in a path; ? matches one

### Working with programs

• General syntax: [PROGRAM NAME OR PATH] [ARGUMENT 1] [ARGUMENT 2] [...] (quote arguments if they have spaces)
• For help: [PROGRAM] -h or [PROGRAM] --help or man [PROGRAM]
• Redirecting output to file: [PROGRAM] [ARGUMENTS] > [OUTPUT FILE]
• Piping output to another program: [PROGRAM 1] [ARGUMENTS] | [PROGRAM 2] [ARGUMENTS]

• View zipped file: zcat [FILE] | less (gzcat on OS X)
• Redirect output to log: [PROGRAM WITH OUTPUT] > [OUTPUT FILE]
• Redirect output and errors to log: [PROGRAM WITH OUTPUT] 2>&1 logfile.log

Writing shell scripts:

• Begin the script with “shebang” line: #!/bin/bash
• Use # for comments
• Run with bash [SCRIPT PATH] or [SCRIPT PATH] (with ./ if in current directory)
• If running as [SCRIPT PATH], use chmod +x [SCRIPT PATH] to set permissions

## Why should I use the command line?

Like the desktop interface you use on your computer every day, the command line allows you to work with files and programs. Though the commands might feel clunky at first, you’ll find that using the command line offers several important benefits:

• Power. Many tools can only be used through the command line. This is particularly the case for bioinformatics tools–to use many cutting-edge bioinformatic algorithms, you’ll need to run them in the command line.
• Flexibility. With basic familiarity with the command line, you can combine different tools and automate parts of your analyses.
• Reproducibility. With its simple interface and the ability to save lists of commands as scripts, the command line is a powerful tool for reproducible analysis.
• Working remotely. If you want to run analyses remotely (for instance, on a more powerful machine or a computing cluster) you’ll often need to work in the command line.

## What is Unix? What is the command-line? What is bash?

Unix is a family of operating systems which share a set of tools and a system for organizing files and resources. Both Linux and OS X are “Unix-like” operating systems, and the tools we’ll use in this tutorial should work on both.

When using the command line, you enter commands which are interpreted by a shell or command-line interpreter. The shell reads a command input by the user, runs the action, then repeats this cycle indefinitely.

bash is the most commonly used shell and is often the default, so this tutorial focuses on bash syntax. We’ll just go over basic commands, but bash has its own powerful (but clunky) programming language; I recommend checking some more advanced features (for loops, if statements, and variables come in handy quite often).

## 1. Getting started: navigating the file system

Let’s get started. Open up a command line terminal and type ls, then press enter. You will see a list of files and folders. On my computer, I see:

Desktop
Documents
Music
Pictures
Videos


The ls command (short for “list”) lists files and folders in the current directory. You can change directories with cd (short for “change directories”). Enter the command cd Desktop (use the name of another directory if you don’t have a Desktop folder). Then, use ls again. You should see the same list of files.

How do we go back? Unix uses .. to refer to the directory above the one we’re currently in. So, enter cd .., then check that we’re back in our starting location by listing the files with the ls command.

You’ve seen how to do some basic navigation. Next, we’ll learn how Unix uses paths to specify locations. Desktop and .. are both examples of paths. As you may have seen before, a path is a list of folders, separated with /. It is very important to distinguish between absolute and relative paths.

### Absolute paths

An absolute path refers unambiguously to a single location. Just as the address “1600 Pennsylvania Ave NW, Washington, DC, USA” refers to a particular building, an absolute path like /home/joe/Documents/file.txt refers to a specific file. Absolute paths begin with a forward slash / or a tilde ~. This is because in Unix operating systems (including OS X and Linux), all files live under a root directory called /.

The tilde ~ is a special character in absolute paths. Paths beginning with ~ refer to the current user’s home directory (often something like /home/USERNAME). Usually, when starting a session in the command line, you begin in your home directory.

### Relative paths

A relative path gives a location relative to the current location. Just as the phrase “the person on my right” can refer to different people depending on where the speaker is, a relative path like Documents/file.txt only makes sense if you know the current directory (the working directory). If you are in the directory /home/joe/, the path Documents/file.txt refers to a file named file.txt inside a directory named Documents which is in the current directory–that is, a file with the absolute path /home/joe/Documents/file.txt. If a path doesn’t start with / or ~, it is a relative path.

In relative paths, .. and . have special meanings. The symbol .. is very useful; it means “the directory above.” So, if we’re in /home/joe/Documents/, ../ refers to /home/joe/ and ../../ refers to /home/. It can also show up in the middle of paths: Documents/../Files/ would refer to /home/joe/Files/.

Paths starting with ./ refer to things inside the current folder, so ./Documents is the same as Documents. This may seem pointless, but this is actually useful in some cases (e.g. when we run programs, as you’ll see later).

With both types of path, be careful about spaces and certain other special characters (including &'"\$<>()|"). The interpreter treats spaces as separators, so a path like /home/joe/Documents/New File.txt looks like two separate parts: /home/joe/Documents/New and File.txt. Paths with special characters need to be quoted: '/home/joe/Documents/New File.txt'. You can also put a backslash before the special character: /home/joe/Documents/My\ File.txt

One final note: if a path ends with a /, it must refer to a directory. However, the reverse is not true: a path not ending in / might still be a directory.

### Putting it together

Using your knowledge of paths, you can navigate the entire file system. Usually, relative and absolute paths can be used interchangably.

Enter pwd (short for “print working directory”; it has nothing to do with passwords). This tells you your current location as an absolute path; mine is /home/joe/. pwd is useful when you forget where you currently are!

Earlier, we used cd to navigate one directory at a time, but it is actually more powerful. You can give it an absolute path: try using cd / to go to /, the root directory, and using ls to list files. Then, go back to the original directory (use the absolute path previously printed by pwd). Practice using cd with some different absolute and relative paths.

The ls command also has other useful abilities. You can give it a path to list the contents of that path: for example, ls /home/joe/Desktop. Want to know how big your files are? Use ls -lh to see this information. (Try including a path after ls -lh!)

Finally, you can make directories with the mkdir command and remove an empty directory with the rmdir command. Try this out by making a directory for your command-line learning efforts: I put mine at ~/Documents/command_line_demo. Make a directory inside that directory called junk, then remove the directory with rmdir.

### Summary: navigating the file system

• Change directories: cd [PATH]
• List files in directory: ls (current directory) or ls [PATH] (another directory)
• See file sizes in a directory: ls -lh [PATH]
• Print current directory: pwd
• Make a directory: mkdir [PATH]
• Remove an empty directory: rmdir [PATH]

## 2. Working with files

Now we know how to move around the filesystem. However, that isn’t very useful if it’s all we can do! To do work using the command line, we have to know how to work with files.

Many of the files you work with will be downloaded from the web. It’s often handy to use the command line to download files. The wget tool will do this. Let’s download an ebook of On the Origin of Species from the Project Gutenberg website using the command:

wget 'http://www.gutenberg.org/cache/epub/2009/pg2009.txt'


On OS X, you will have to use:

curl -o pg2009.txt 'http://www.gutenberg.org/cache/epub/2009/pg2009.txt'


Is this the right file? We could open it up in Microsoft Word to see, but let’s stick to command line tools. The cat command prints the contents of a text file. Let’s try it:

cat pg2009.txt


Wow, that printed the entire text of On the Origin of Species! There are other ways to view this file without printing all of it. Try:

head -10 pg2009.txt


This should print the first 10 lines of our file. Finally, for something fancy, try the less viewer:

less pg2009.txt


This lets you scroll up and down through the file, all in the command line! Type the letter ‘q’ to exit.

### Moving, copying, and deleting files

Having our Darwin ebook is nice, but pg2009.txt isn’t an informative title. We could have used command line options to download it to a different location: wget -O [OUTPUT FILE] [URL] or curl -o [OUTPUT FILE] [URL], but this is a good opportunity to learn how to rename files. Let’s rename the file using the mv (“move”) command:

mv pg2009.txt 'Origin of Species.txt'


Notice the single quotes around the filename! This is necessary because there are spaces in the new path. The command mv pg2009.txt Origin\ of\ Species.txt would also have worked. Also, though we’ve just used mv to rename a file, mv will also move files between directories if the new path is a directory (test/) or points to a location in a different directory (test/'Origin of Species.txt'):

mkdir test/
mv 'Origin of Species.txt' test/
# Equivalent to the following:
# mv 'Origin of Species.txt' test/'Origin of Species.txt'


Now try your hand at moving the file back to our working directory (hint: ./ is the relative path for the current directory).

Copying books is hard, but copying ebooks is trivial. Let’s make a new copy of Darwin’s book using the cp (copy) command:

cp 'Origin of Species.txt' darwin.txt


Use the head or less command to check that darwin.txt has the same text.

I’m getting overwhelmed by all these files; aren’t you? Let’s delete the extra copy using the rm command:

rm darwin.txt


Be VERY careful using the rm command. Unlike deleting things in the Finder, rm doesn’t send things to a trash folder. Once you rm a file, it’s gone forever! If you’re nervous about this, you can install and use the trash tool for OS X or trash-cli for some Linux systems.

rm will also delete entire directories if you need it to (even if there are files inside). First, make a folder with some files in it:

mkdir Library/
cp 'Origin of Species.txt' Library/darwin.txt


You can check that this worked with the ls commands. To delete the directory, use rm -r (“remove, recursive”).

rm -r Library/


Notice that rm -r looks like ls -lh: both have a command (rm, ls) followed by some options (-r, -lh). Another useful command is cp -r (“copy, recursive”), which copies an entire directory (including any files inside it). We’ll learn more about options when we discuss running programs.

### Other tricks for files

#### Creating files

Want to create a blank file? Do the following:

touch file.txt


#### Editing text

Often you’ll want to make small edits to text from within the terminal. There are many tools for this but I find that nano is the simplest and easiest to use. Try:

nano 'Origin of Species.txt'


To exit, hit Ctrl + X; you’ll be given options to save or discard changes. This and other commands are at the bottom of the screen (^X is Ctrl + X and so on) if you forget.

If you give nano the name of a file that doesn’t exist yet, it will create that file for you.

Looking for text in a file is often useful for bioinformatics. However, if you’ve ever accidentally opened a genome in Microsoft Word, you might know that this could be painfully slow. The grep tool is very fast and incredibly useful for bioinformatics! As a demo, we can find out what Darwin has to say about fungus:

 grep 'fungus' 'Origin of Species.txt'


(The first thing after grep is what you’re searching for; the next is the file you’re looking in. I mix up the order of these two all the time.)

Not bad! Darwin uses that word twice: once in the line “…the water or some parasitic fungus is infinitely more numerous in…” and again in “…fungus exceeds its allies in the above respects, it will then be…”

Even cooler: we can find the context of those lines. -A [NUMBER] and -B [NUMBER] tell grep how many lines before and after a match to print. So:

grep 'fungus' 'Origin of Species.txt' -A 10 -B 10


This shows us the only passage in the Origin where Darwin discusses fungi–a very interesting paragraph about niches and resource partitioning. (But do closely related species actually compete more strongly?)

One of my favorite Unix features is the ability to link files. Instead of copying a file, you can make a symbolic link to the file that you can read from and write to just like a real file. To do this, use ln -s [PATH] (“link, symbolic”):

ln -s 'Origin of Species.txt' 'symbolic_link.txt'


View the file (head or less) to see that the contents are the same. You need to be careful with symbolic links, however. First, making changes in the linked version will also change the original file–I have accidentally overwritten data this way. Second, if your link uses a relative path and you move the directory containing the link, the link will no longer point to a valid file.

You can also make symbolic links to directories, which comes in handy pretty often.

#### Wildcards

It can be frustrating using commands like mv or rm on many similar files. Say we have a bunch of text files:

cp 'Origin of Species.txt' junk1.txt
cp 'Origin of Species.txt' junk2.txt
cp 'Origin of Species.txt' junk03.txt
cp 'Origin of Species.txt' more_junk.txt


We could delete each file separately, but this takes too long. Instead, we can use the wildcards * and ?. * can match any number of characters, while ? can match only one character. So, this command will delete only junk1.txt and junk2.txt:

rm junk?.txt


This command will delete junk1.txt, junk2.txt, and junk03.txt:

rm junk*.txt


Finally, this command will delete all four files:

rm *junk*.txt


This command successfully matches junk1.txt, etc. because * is allowed to match zero characters.

It’s important to understand how the shell actually processes wildcards. Under the hood, whenever it sees part of a command with * or ?, it will replace that with a list of valid paths matching that command. So, when we typed rm junk*.txt, the shell converted it to this command:

rm junk1.txt junk2.txt junk03.txt


This is subtle, but very important to understand. Some commands won’t take more than one path, so using a wildcard won’t allow you to run them on multiple files. Also, using wildcards can lead to unexpected effects. What does the following command do?

mv junk?.txt


mv doesn’t work with one path, but this command actually does something. Why? The shell expands it to mv junk1.txt junk2.txt, and overwrites junk2.txt with junk1.txt. I’ve accidentally overwritten data this way!

### Summary: working with files

Basic operations:

• Move/rename a file: mv [FILE] [NEW PATH]
• Delete a file (permanently!): rm [FILE]
• Delete an entire directory (permanently!): rm -r [DIRECTORY]
• Copy a file: cp [FILE] [NEW PATH]
• Copy an entire directory: cp -r [DIRECTORY] [NEW PATH]

Useful tricks:

• Download a file: wget -O [OUTPUT FILE] [URL] or curl -o [OUTPUT FILE] [URL] (wget may not be available in OS X)
• View a file: head -[NUMBER OF LINES] [FILE] or less [FILE]
• Create a blank file: touch [FILE]
• Edit a file: nano [FILE]
• Search within a file: grep [QUERY] [FILE]
• Create a symbolic link to a file: ln -s [FILE] [NEW LINK]
• Wildcards: * matches any number of characters in a path; ? matches one

## 3. Working with programs

Okay, now we have some familiarity with moving around in directories and with manipulating files. Now, we’re ready to start actually doing stuff–and for that, we need programs. We’ll learn to give input and options to programs, and to work with output.

To run a program in the command line, you type the name of the program, or a path to the executable file that runs the program. After that, include any arguments (information about what to do) that the program expects. All parts of this command should be separated with spaces–this is why you had to quote the filename if it contained spaces (to prevent it being interpreted as multiple arguments). When we run grep 'fungus' 'Origin of Species.txt', Unix interprets this as three parts: grep fungus Origin of Species.txt; grep is the program and the other two parts are arguments.

### Figuring out program usage

Many of the commands we’ve seen take arguments that modify their behavior– think ls -lh and cp -r. Often, these arguments (“flags”) begin with one or two hyphens. When using a new program, we have to figure out which flags we need to do the task we want.

Fortunately, command-line programs worth their salt come with built-in help. This can usually be found with [PROGRAM] -h or [PROGRAM] --help, or by entering man [PROGRAM] (for “manual”).

We’ll use the wc command as an example. Let’s say we’re interested in how many lines are in our Origin of Species.txt file. The wc (“word count”) program can count lines, but how? Let’s read the help. Try the three commands and see which one works:

wc -h
wc --help
man wc


Which option gives us the line count? Count the lines using this option. You should come up with around twenty thousand lines!

### Redirecting output

Many programs can take their input from other programs and give their output to other programs. You can take advantage of this to control where output goes.

We saw that grep can be used to be search lines. Let’s say we’d like to write the fungus passage we found earlier to a file. For this we use >, which redirects output that would be written to the command line to a file:

grep 'fungus' 'Origin of Species.txt' -A 10 -B 10 > 'darwin_on_fungi.txt'


Run the command and check this file. Redirecting can be useful to save error messages to a log file. For this, you’ll want to use 2>&1, which covers error messages as well as regular output:

[PROGRAM WITH OUTPUT] 2>&1 logfile.log


You can also send the output of programs through other programs using a |, which is known as piping output. Let’s use the wc program to count the lines in the passage, without having to save it to a file:

grep 'fungus' 'Origin of Species.txt' -A 10 -B 10 | wc -l


You can pipe multiple times and even combine with redirecting:

grep 'fungus' 'Origin of Species.txt' | wc -l > 'darwin_on_fungi_line_count.txt'


This finds the lines, counts them, then writes the number to a file.

This is one of the most powerful features of the Unix command line. One of the most useful commands when processing nucleotide data is zcat [FILE] | wc -l, (gzcat if on OS X) which uses zcat to uncompress the data in a file, then sends it to wc -l to count the lines. You can also use zcat [FILE] | less to view a zipped file without uncompressing it.

### Your own programs: simple shell scripts

You can also run custom programs. One useful example is storing your command line commands in a file called a shell script.

Let’s say we want some easier way to run the command ls -lh ~/ (checking contents of home directory and sizes). Create a file called check_home.sh in you working directory and edit it (using a text editor, or the nano command for extra credit). Enter the following text:

#!/bin/bash
ls -lh ~/


The first line is a standard way to start bash scripts, called a “shebang” line (“hash” for # + “bang” for !). The part after #! is the program that should be used to run the file, in this case bash (the program that processes command line input).

Save the file and run it like this:

bash check_home.sh


This should list information about the file in your home directory. But let’s say we want to run the check_home.sh program without having to type bash before it. Let’s try

check_home.sh


This fails, saying check_home.sh: command not found. This is because Unix won’t run a program in your current directory unless you add ./ before the name. This is a security feature: imagine if someone tricked you into downloading a file called cd. So, let’s try:

./check_home.sh


Oh no, another error: bash: ./check_home.sh: Permission denied. What happened? Unix won’t run a file unless you give that file execute permission. This is another security feature. The chmod command can be used to add this permission:

chmod +x check_home.sh


Now, it should work as intended! Run it again using:

./check_home.sh


Finally, let’s say we want to add information about what the script’s doing. Update the file so it says:

#!/bin/bash
# List the contents of our home directory
ls -lh ~/


The second line beginning with # is a comment; it tells bash not to run the rest of the line. Comments can also go at the end of a line of code:

#!/bin/bash
# List the contents of our home directory
ls -lh ~/ # This line has a end comment


Run the commented script to see that it works properly. Comments are a great way to make your shell scripts clearer.

You’ve saved the commands for a simple “analysis” into a shell script! This is really convenient for doing complex analyses: just put all the commands into a shell script, and run the whole script.

Bash (this particular type of shell script) is actually its own programming language, and has variables, loops, and other features you’d expect. You can implement complex logic in your scripts if you need.

### Summary: working with programs

• General syntax: [PROGRAM NAME OR PATH] [ARGUMENT 1] [ARGUMENT 2] [...] (quote arguments if they have spaces)
• For help: [PROGRAM] -h or [PROGRAM] --help or man [PROGRAM]
• Redirecting output to file: [PROGRAM] [ARGUMENTS] > [OUTPUT FILE]
• Piping output to another program: [PROGRAM 1] [ARGUMENTS] | [PROGRAM 2] [ARGUMENTS]

Useful pipes/redirects:

• Count lines in zipped file: zcat [FILE] | wc -l (gzcat on OS X)
• View zipped file: zcat [FILE] | less (gzcat on OS X)
• Redirect output to log: [PROGRAM WITH OUTPUT] > [OUTPUT FILE]
• Redirect output and errors to log: [PROGRAM WITH OUTPUT] 2>&1 logfile.log

Writing shell scripts:

• Begin the script with “shebang” line: #!/bin/bash
• Use # for comments
• Run with bash [SCRIPT PATH] or [SCRIPT PATH] (with ./ if in current directory)
• If running as [SCRIPT PATH], use chmod +x [SCRIPT PATH] to set permissions