Note that commands in Linux have flags. The ls command we used earlier has a lot of other options - to see the options available, type:
man ls
man is a command to display the manual for a specific command. To exit the page, type q.
Now that we know that, we can see more from ls - by using the long format for display. For example, we can run a list in long format for the /tmp/test directory we made earlier, by running:
ls -l /tmp/test/
Which should output something like:
drwxr-xr-x 3 john.contad wheel 102 Mar 1 10:40 .
drwxrwxrwt 14 root wheel 476 Mar 1 10:40 ..
-rw-r--r-- 1 john.contad wheel 0 Mar 1 10:40 test.txt
On the first column, you have the permissions for the object, followed by the owner, the group, and last time the file was modified or created. The first column is particularly helpful, because this lists what we can do with the file.
Note that the format stands for three sets of read, write, and execute permissions for the user, the group, and everyone - respectively.
For example, changing test.txt to be readable and writable and executable by the user, we'll need to modify the bits in the file to be like rwxr--r--. In binary, it would look like:
111 100 100
Which in real numbers is:
7 4 4
When we execute the command to change permissions ( chmod ), we'll need to run:
chmod 744 /tmp/test/test.txt
And if we're changing test.txt to be readable and executable by the user and group, we'll need to modify the bits in the file to be like: r-xr--r--. In binary, it would look like:
101 101 100
Which in real numbers is:
5 5 4
When we execute the command to change permissions ( chmod ), we'll need to run:
chmod 554 /tmp/test/test.txt
The output of commands can be redirected to a file. To do so, simply use the > character like so:
ls > /tmp/banana.txt
To see the contents of the file, you can use the cat command, like so:
cat /tmp/banana.txt
Note that the > character overwrites the contents of the destination file. If you were to run a redirection to the same file, the contents would be replaced. For example:
echo BANANA > /tmp/banana.txt
cat /tmp/banana.txt
To append output to the end of the file, simply use >> instead of >. For example:
echo BANANA > /tmp/banana.txt
echo BANANA >> /tmp/banana.txt
echo BANANA >> /tmp/banana.txt
Would make the banana.txt file contain:
BANANA
BANANA
BANANA
Note that it is possible to redirect files to commands as well. For example, if you created a file that contained a list of fruits like so:
echo apple > fruit.txt
echo guava >> fruit.txt
echo banana >> fruit.txt
echo grapes >> fruit.txt
You can redirect the file to a command, like so:
sort < fruit.txt
You can also redirect the output of the previous redirection like so:
sort < fruit.txt > sortedfruit.txt
You can use pipes ( | ) to redirect output to commands as well. For example, you can pipe the output of ls to sort like so:
ls / | sort
This should give you an alphabetized list. You can also run an equivalent of the standard input redirection that you previously ran, by outputting the contents of the file using cat:
cat fruit.txt | sort
Try to create a newline-separated file like so:
echo banana > fruit.txt
echo banana >> fruit.txt
echo banana >> fruit.txt
echo banana >> fruit.txt
echo apple >> fruit.txt
echo apple >> fruit.txt
echo grapes >> fruit.txt
echo guavas >> fruit.txt
echo dates >> fruit.txt
This should create a file that looks like this:
$ cat fruit.txt
banana
banana
banana
banana
apple
apple
grapes
guavas
dates
How would you use pipes ( | ) to run this command through sort and uniq, so that it's alphabetized and deduplicated?