3  Tips and tricks

We already know one shortcut to go quickly to the home directory:

cd ~

The single dot . is a shortcut for the current working directory.

cd .
ls .

3.1 Wild cards

Navigate to the ~/training/training-data/references directory. This directory contains five references. If we type ls, we will see that there are a bunch of files. By default, ls lists all of the files in a given directory. The * character is a shortcut for “everything”. Thus, if you enter ls *, you will see all of the contents of a given directory. Now try this command:

ls *.bib

Now let’s list all the files that start with ‘cit’

ls cit*

3.2 Tab Completion

Navigate to the ~/training/ directory.

Typing out directory names can waste a lot of time. When you start typing out the name of a directory, then hit the tab key, the shell will try to fill in the rest of the directory name.

For example, enter:

cd t<tab>

The shell will fill in the rest of the directory name for training. Now go to training/training-data.

ls r<tab><tab>

When you hit the first tab, nothing happens. The reason is that there are multiple directories in the home directory which start with r. Thus, the shell does not know which one to fill in. When you hit tab again, the shell will list the possible choices.

Tab completion can also fill in the names of programs. For example, enter e<tab><tab>. You will see the name of every program that starts with an e. One of those is echo. If you enter ec<tab> you will see that tab completion works.

3.3 History

You can easily access previous commands. Hit the up arrow. Hit it again. You can step backwards through your command history. The down arrow takes your forwards in the command history.

^-C will cancel the command you are writing, and give you a fresh prompt.

^-R (arrow pointing up) will do a reverse-search through your command history. This is very useful.

You can also review your recent commands with the history command. Just enter:

history

to see a numbered list of recent commands, including this just issues history command. You can reuse one of these commands directly by referring to the number of that command.

If your history looked like this:

258 cd ..
259 ls *
260 cd ~

then you could repeat command #259 by simply entering:

!259

(that’s an exclamation mark).

3.4 Which program?

Commands like ls, rm, echo, and cd are just ordinary programs on the computer. A program is just a file that you can execute. The program which tells you the location of a particular program. For example:

which ls

Will return /bin/ls. Thus, we can see that ls is a program that sits inside of the /bin directory. Now enter:

which find

You will see that find is a program that sits inside of the /usr/bin directory.

When there are no / characters, the shell assumes you want to look in one of the default places for the program.

3.5 Inspecting 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?

The easiest way to examine a file is to just print out all of the contents using the program cat. Enter the following command:

cat file.txt

This file is empty because we created this file just earlier and we did not write anything in it. If we navigate to ~/training/training-data/, we can check the file called tsl.txt”

cat tsl.txt

Now let’s do:

cat tsl.txt tsl.txt

It will print out the contents of tsl.txt twice. cat actually stands for “concatenate” and takes a list of file names and writes them out one after another. 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. Enter the following command:

less dict.txt

less opens the file, and lets you navigate through it. The commands are identical to the man program.

Some commands in less

key	action
“space”	to go forward
“b”	to go backwarsd
“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. Just hit the / key to begin a search. Enter the name of the word you would like to search for and hit enter. It will jump to the next location where that word is found. Try searching the dict.txt file for the word “cat”. If you hit / then enter, less will just repeat the previous search. less searches from the current location and works its way forward. If you are at the end of the file and search for the word “cat”, less will not find it. You need to go to the beginning of the file and search.

Remember, the man program actually uses less internally and therefore uses the same commands, so you can search documentation using / as well!

Another option for examining the content of a file is to look at the first/last few lines with the command head and tail.

head dict.txt

tail dict.txt

3.6 Catching patterns

Now, we would like to see all the word containing the letter “e” in the dictionary. We can use the command grep:

grep 'e' dict.txt

Now let’s grab all the words in the dictionary that contain the pattern “cat”

grep 'cat' dict.txt

Now we can use more complicated patterns called ‘regular expressions’ to catch the lines we are interested in. For example, all the words in the dictionary starting with ‘e’.

grep '^e' dict.txt

or all the words in the dictionary ending with ‘e’.

grep 'e$' dict.txt

3.7 Redirection

How about keeping a record of all the words we found ending with the letter ‘e’:

grep 'e$' dict.txt > ending.txt

Let’s check the content of this new file ending.txt.

head ending.txt
less ending.txt
cat ending.txt

3.8 Creating, moving, copying, and removing

Now let’s backup our work.

mkdir backup

We will now move our newly created file to the backup directory:

mv ending.txt backup/

We can also copy the entire dictionary dict.txt to the backup directory.

cp dict.txt backup/

We insure that the files have been either moved or copied.

ls backup/

To clarify within the backup directory that it contains a copy of the directory. There is no such command as rename, but we can use the mv command instead:

mv backup/dict.txt backup/dict-copy.txt

We can now check if the change was made:

ls backup/

For extra safety, we want to hide th backup directory In this case, we create a copy of our backup directory with the cp command. By default, cp works for files (not directory/folders), consequently we must use the argument -r, which stands for recursive, to copy the entire directory (and its content).

cp -r backup/ .backup/

Note

In Unix-like operating systems, any file or folder that starts with a dot character (for example, /home/user/. config), commonly called a dot file or dotfile, is to be treated as hidden – that is, the ls command does not display them unless the -a or -A flags ( ls -a or ls -A ) are used.

Now we can remove the regular backup directory:

rm backup

3.9 Count the words

The wc program (word count) counts the number of lines, words, and characters in one or more files.

Make sure you are in the training-data directory, then enter the following command:

wc tsl.txt

For each of the files indicated, wc prints a line with three numbers. The first is the number of lines in that file. The second is the number of words. Finally, the total number of characters is indicated. The final line contains this information summed over all of the files.

wc *.txt

3.10 The awesome power of the Pipe

Let’s say we want to know the number of words containing the pattern ‘ef’ in the dictionary. We know both command grep and wc, and we execute them one after the other using the pipe |.

grep 'ef' dict.txt | wc

Let’s see if the word ‘effector’ is in the dictionary

grep 'effector' dict.txt

We can open and edit the file within the Terminal, using a program called nano:

nano dict.txt

Add the word ‘effector’ at the beginning of the file and use the Ctrl-x command to save and exit the text editor.

We verify that the change was made:

head dict.txt

Indeed, the word effector is now at the beginning of the dictionary. Great, but a dictionary should be sorted by alphabetical order. Let’s reorder the words in the dictionary using the command sort

sort dict.txt > sorted-dict.txt

We verify that the change was made:

head sorted-dict.txt
grep 'effector' sorted-dict.txt
grep '^effect' sorted-dict.txt

We can now rename (and replace) the dictionary:

mv sorted-dict.txt dict.txt