Working with Files and Directories
Last updated on 2024-11-14 | Edit this page
Overview
Questions
- How can I view and search file contents?
- How can I create, copy and delete files and directories?
- How can I control who has permission to modify a file?
- How can I repeat recently used commands?
Objectives
- View, search within, copy, move, and rename files. Create new directories.
- Use wildcards (
*
) to perform operations on multiple files. - Make a file read only.
- Use the
history
command to view and repeat recently used commands.
Working with Files
Our data set: FASTQ files
Now that we know how to navigate around our directory structure,
let’s start working with our sequencing files. We did a sequencing
experiment and have two results files, which are stored in our
untrimmed_fastq
directory.
Wildcards
Navigate to your untrimmed_fastq
directory:
We are interested in looking at the FASTQ files in this directory. We can list all files with the .fastq extension using the command:
OUTPUT
SRR097977.fastq SRR098026.fastq
The *
character is a special type of character called a
wildcard, which can be used to represent any number of any type of
character. Thus, *.fastq
matches every file that ends with
.fastq
.
This command:
OUTPUT
SRR097977.fastq
lists only the file that ends with 977.fastq
.
This command:
OUTPUT
/usr/bin/gettext.sh /usr/bin/ibdiagm.sh /usr/bin/mft_uninstall.sh /usr/bin/unix-lpr.sh
/usr/bin/gflags_completions.sh /usr/bin/lesspipe.sh /usr/bin/mlnx_interface_mgr.sh
/usr/bin/gvmap.sh /usr/bin/lprsetup.sh /usr/bin/setup-nsssysinit.sh
Lists every file in /usr/bin
that ends in the characters
.sh
. Note that the output displays full
paths to files, since each result starts with /
.
Exercise
Do each of the following tasks from your current directory using a
single ls
command for each:
- List all of the files in
/usr/bin
that start with the letter ‘c’. - List all of the files in
/usr/bin
that contain the letter ‘a’. - List all of the files in
/usr/bin
that end with the letter ‘o’.
Bonus: List all of the files in /usr/bin
that contain
the letter ‘a’ or the letter ‘c’.
Hint: The bonus question requires a Unix wildcard that we haven’t talked about yet. Try searching the internet for information about Unix wildcards to find what you need to solve the bonus problem.
ls /usr/bin/c*
ls /usr/bin/*a*
-
ls /usr/bin/*o
Bonus:ls /usr/bin/*[ac]*
Exercise
echo
is a built-in shell command that writes its
arguments, like a line of text to standard output. The echo
command can also be used with pattern matching characters, such as
wildcard characters. Here we will use the echo
command to
see how the wildcard character is interpreted by the shell.
OUTPUT
SRR097977.fastq SRR098026.fastq
The *
is expanded to include any file that ends with
.fastq
. We can see that the output of
echo *.fastq
is the same as that of
ls *.fastq
.
What would the output look like if the wildcard could not be
matched? Compare the outputs of echo *.missing
and
ls *.missing
.
Command History
If you want to repeat a command that you’ve run recently, you can access previous commands using the up arrow on your keyboard to go back to the most recent command. Likewise, the down arrow takes you forward in the command history.
A few more useful shortcuts:
- Ctrl+C will cancel the command you are writing, and give you a fresh prompt.
- Ctrl+R will do a reverse-search through your command history. This is very useful.
-
Ctrl+L or the
clear
command will clear your screen.
You can also review your recent commands with the
history
command, by entering:
to see a numbered list of recent commands. You can reuse one of these commands directly by referring to the number of that command.
For example, if your history looked like this:
OUTPUT
259 ls *
260 ls /usr/bin/*.sh
261 ls *R1*fastq
then you could repeat command #260 by entering:
Type !
(exclamation point) and then the number of the
command from your history. You will be glad you learned this when you
need to re-run very complicated commands. For more information on
advanced usage of history
, read section 9.3 of Bash
manual.
Exercise
Find the line number in your history for the command that listed all
the .sh files in /usr/bin
. Rerun that command.
First type history
. Then use !
followed by
the line number to rerun that command.
Examining Files
We now know how to switch directories, run programs, and look at the contents of directories, but how do we look at the contents of files?
One way to examine a file is to print out all of the contents using
the program cat
.
Enter the following command from within the
untrimmed_fastq
directory:
This will print out all of the contents of the
SRR098026.fastq
to the screen.
Exercise
- Print out the contents of the
~/obss_2024/commandline/shell_data/untrimmed_fastq/SRR097977.fastq
file. What is the last line of the file? - From your home directory, and without changing directories, use one
short command to print the contents of all of the files in the
~/obss_2024/commandline/shell_data/untrimmed_fastq
directory.
- The last line of the file is
C:CCC::CCCCCCCC<8?6A:C28C<608'&&&,'$
. cat ~/obss_2024/commandline/shell_data/untrimmed_fastq/*
cat
is a terrific program, but when the file is really
big, it can be annoying to use. The program, less
, is
useful for this case. less
opens the file as read only, and
lets you navigate through it. The navigation commands are identical to
the man
program.
Enter the following command:
Some navigation commands in less
:
key | action |
---|---|
Space | to go forward |
b | to go backward |
g | to go to the beginning |
G | to go to the end |
q | to quit |
less
also gives you a way of searching through files.
Use the “/” key to begin a search. Enter the word you would like to
search for and press enter
. The screen will jump to the
next location where that word is found.
Shortcut: If you hit “/” then “enter”,
less
will repeat the previous search. less
searches from the current location and works its way forward. Scroll up
a couple lines on your terminal to verify you are at the beginning of
the file. Note, if you are at the end of the file and search for the
sequence “CAA”, less
will not find it. You either need to
go to the beginning of the file (by typing g
) and search
again using /
or you can use ?
to search
backwards in the same way you used /
previously.
For instance, let’s search forward for the sequence
TTTTT
in our file. You can see that we go right to that
sequence, what it looks like, and where it is in the file. If you
continue to type /
and hit return, you will move forward to
the next instance of this sequence motif. If you instead type
?
and hit return, you will search backwards and move up the
file to previous examples of this motif.
Exercise
What are the next three nucleotides (characters) after the first instance of the sequence quoted above?
CAC
Remember, the man
program actually uses
less
internally and therefore uses the same commands, so
you can search documentation using “/” as well!
There’s another way that we can look at files, and in this case, just look at part of them. This can be particularly useful if we just want to see the beginning or end of the file, or see how it’s formatted.
The commands are head
and tail
and they let
you look at the beginning and end of a file, respectively.
OUTPUT
@SRR098026.1 HWUSI-EAS1599_1:2:1:0:968 length=35
NNNNNNNNNNNNNNNNCNNNNNNNNNNNNNNNNNN
+SRR098026.1 HWUSI-EAS1599_1:2:1:0:968 length=35
!!!!!!!!!!!!!!!!#!!!!!!!!!!!!!!!!!!
@SRR098026.2 HWUSI-EAS1599_1:2:1:0:312 length=35
NNNNNNNNNNNNNNNNANNNNNNNNNNNNNNNNNN
+SRR098026.2 HWUSI-EAS1599_1:2:1:0:312 length=35
!!!!!!!!!!!!!!!!#!!!!!!!!!!!!!!!!!!
@SRR098026.3 HWUSI-EAS1599_1:2:1:0:570 length=35
NNNNNNNNNNNNNNNNANNNNNNNNNNNNNNNNNN
OUTPUT
+SRR098026.247 HWUSI-EAS1599_1:2:1:2:1311 length=35
#!##!#################!!!!!!!######
@SRR098026.248 HWUSI-EAS1599_1:2:1:2:118 length=35
GNTGNGGTCATCATACGCGCCCNNNNNNNGGCATG
+SRR098026.248 HWUSI-EAS1599_1:2:1:2:118 length=35
B!;?!A=5922:##########!!!!!!!######
@SRR098026.249 HWUSI-EAS1599_1:2:1:2:1057 length=35
CNCTNTATGCGTACGGCAGTGANNNNNNNGGAGAT
+SRR098026.249 HWUSI-EAS1599_1:2:1:2:1057 length=35
A!@B!BBB@ABAB#########!!!!!!!######
The -n
option to either of these commands can be used to
print the first or last n
lines of a file.
OUTPUT
@SRR098026.1 HWUSI-EAS1599_1:2:1:0:968 length=35
OUTPUT
A!@B!BBB@ABAB#########!!!!!!!######
Details on the FASTQ format
Although it looks complicated (and it is), it’s easy to understand the fastq format with a little decoding. Some rules about the format include…
Line | Description |
---|---|
1 | Always begins with ‘@’ and then information about the read |
2 | The actual DNA sequence |
3 | Always begins with a ‘+’ and sometimes the same info in line 1 |
4 | Has a string of characters which represent the quality scores; must have same number of characters as line 2 |
We can view the first complete read in one of the files in our
dataset by using head
to look at the first four lines.
OUTPUT
@SRR098026.1 HWUSI-EAS1599_1:2:1:0:968 length=35
NNNNNNNNNNNNNNNNCNNNNNNNNNNNNNNNNNN
+SRR098026.1 HWUSI-EAS1599_1:2:1:0:968 length=35
!!!!!!!!!!!!!!!!#!!!!!!!!!!!!!!!!!!
All but one of the nucleotides in this read are unknown
(N
). This is a pretty bad read! Line 4 shows the quality
for each nucleotide in the read. We’ll cover the Fastq format more in
depth tomorrow in when we look at assessing
read quality in the DNA variant calling workshop.
Creating, moving, copying, and removing
Now we can move around in the file structure, look at files, and search files. But what if we want to copy files or move them around or get rid of them? Most of the time, you can do these sorts of file manipulations without the command line, but there will be some cases (like when you’re working with a remote computer like we are for this lesson) where it will be impossible. You’ll also find that you may be working with hundreds of files and want to do similar manipulations to all of those files. In cases like this, it’s much faster to do these operations at the command line.
Copying Files
When working with computational data, it’s important to keep a safe copy of that data that can’t be accidentally overwritten or deleted. For this lesson, our raw data is our FASTQ files. We don’t want to accidentally change the original files, so we’ll make a copy of them and change the file permissions so that we can read from, but not write to, the files.
First, let’s make a copy of one of our FASTQ files using the
cp
command.
Navigate to the
~/obss_2024/commandline/shell_data/untrimmed_fastq
directory and enter:
OUTPUT
SRR097977.fastq SRR098026-copy.fastq SRR098026.fastq
We now have two copies of the SRR098026.fastq
file, one
of them named SRR098026-copy.fastq
. We’ll move this file to
a new directory called backup
where we’ll store our backup
data files.
Creating Directories
The mkdir
command is used to make a directory. Enter
mkdir
followed by a space, then the directory name you want
to create:
Moving / Renaming
We can now move our backup file to this directory. We can move files
around using the command mv
:
OUTPUT
SRR098026-copy.fastq
The mv
command is also how you rename files. Let’s
rename this file to make it clear that this is a backup:
OUTPUT
SRR098026-backup.fastq
File Permissions
We’ve now made a backup copy of our file, but just because we have two copies, it doesn’t make us safe. We can still accidentally delete or overwrite both copies. To make sure we can’t accidentally mess up this backup file, we’re going to change the permissions on the file so that we’re only allowed to read (i.e. view) the file, not write to it (i.e. make new changes).
View the current permissions on a file using the -l
(long) flag for the ls
command:
OUTPUT
-rw-r--r-- 1 dcuser dcuser 43332 Nov 15 23:02 SRR098026-backup.fastq
The first part of the output for the -l
flag gives you
information about the file’s current permissions. There are ten slots in
the permissions list. The first character in this list is related to
file type, not permissions, so we’ll ignore it for now. The next three
characters relate to the permissions that the file owner has, the next
three relate to the permissions for group members, and the final three
characters specify what other users outside of your group can do with
the file. We’re going to concentrate on the three positions that deal
with your permissions (as the file owner).
Here the three positions that relate to the file owner are
rw-
. The r
means that you have permission to
read the file, the w
indicates that you have permission to
write to (i.e. make changes to) the file, and the third position is a
-
, indicating that you don’t have permission to carry out
the ability encoded by that space (this is the space where
x
or executable ability is stored, we’ll talk more about
this in a later lesson).
Our goal for now is to change permissions on this file so that you no
longer have w
or write permissions. We can do this using
the chmod
(change mode) command and subtracting
(-
) the write permission -w
.
OUTPUT
-r--r--r-- 1 dcuser dcuser 43332 Nov 15 23:02 SRR098026-backup.fastq
Removing
To prove to ourselves that you no longer have the ability to modify
this file, try deleting it with the rm
command:
You’ll be asked if you want to override your file permissions:
OUTPUT
rm: remove write-protected regular file ‘SRR098026-backup.fastq'?
You should enter n
for no. If you enter n
(for no), the file will not be deleted. If you enter y
, you
will delete the file. This gives us an extra measure of security, as
there is one more step between us and deleting our data files.
Important: The rm
command permanently
removes the file. Be careful with this command. It doesn’t just nicely
put the files in the Trash. They’re really gone.
By default, rm
will not delete directories. You can tell
rm
to delete a directory using the -r
(recursive) option. Let’s delete the backup directory we just made.
Enter the following command:
This will delete not only the directory, but all files within the directory. If you have write-protected files in the directory, you will be asked whether you want to override your permission settings.
Exercise
Starting in the
~/obss_2024/commandline/shell_data/untrimmed_fastq/
directory, do the following:
- Make sure that you have deleted your backup directory and all files it contains.
- Create a backup of each of your FASTQ files using
cp
. (Note: You’ll need to do this individually for each of the two FASTQ files. We haven’t learned yet how to do this with a wildcard.) - Use a wildcard to move all of your backup files to a new backup directory.
- Change the permissions on all of your backup files to be write-protected.
rm -r backup
-
cp SRR098026.fastq SRR098026-backup.fastq
andcp SRR097977.fastq SRR097977-backup.fastq
-
mkdir backup
andmv *-backup.fastq backup
-
chmod -w backup/*-backup.fastq
It’s always a good idea to check your work withls -l backup
. You should see something like:
OUTPUT
-r--r--r-- 1 dcuser dcuser 47552 Nov 15 23:06 SRR097977-backup.fastq
-r--r--r-- 1 dcuser dcuser 43332 Nov 15 23:06 SRR098026-backup.fastq
Key Points
- You can view file contents using
less
,cat
,head
ortail
. - The commands
cp
,mv
, andmkdir
are useful for manipulating existing files and creating new directories. - You can view file permissions using
ls -l
and change permissions usingchmod
. - The
history
command and the up arrow on your keyboard can be used to repeat recently used commands.