6.1 Combining Commands

Question 1

A ________ is several commands that are normally written out one-by-one, chained together in order to create a new command.

Answer 1

A **compound command** is several commands that are normally written out one-by-one, chained together in order to create a new command.

Question 2

What are the three components of a command?

Answer 2

program -options arguments

1. The **program**, which is a binary program that you are running.
2. The program’s **options**, which changes the behavior of the program being run in the first part of the command.
3. The **arguments** provided to the program, usually a reference to a directory or file that you want the program to act on.

Question 3

Compound commands typically follow this format:

Answer 3

program -options arguments | program -options | program -options | program -options

Question 4

Breakdown this command:

file $(find / -iname *.txt 2>/dev/null) > ~/Desktop/text_files ; tail ~/Desktop/text_files

Answer 4

• Searches the entire computer for files ending in .txt;
• Verifies that the files found are text files; Ignores any errors it comes across; Creates a list of all found files before saving the list to the desktop
• Finally, it will open the file and print the last ten lines that were added.

Question 5

Explain these command:

1. ls > list.txt
2. > list.txt
3. ls >> list.txt

Answer 5

1. This command takes the output of the ls command and sends it into a new file named list.txt. _If_ the file list.txt already exists, it is overwritten with the output of the ls command.
2. In this case, we didn’t put a command in front of \> so there is no output to send to the list.txt file.
   * However, the file is still written, just with no output. So a blank file is written. _If_ the file list.txt exists, it is overwritten with nothing.
3. >> will append the output of the ls command to the list.txt file.

• If the list.txt file does not exist, it is created.
• Therefore, using \>\> instead of \> is always safer, unless you want the file to be overwritten.

Question 6

What do pipes ( | ) do?

Answer 6

The pipe ( | ) takes the output of one command and sends it to the input of another command.

Question 7

Explain this command:

ls -l | grep *.txt

Answer 7

The pipe ( | ) takes the output of one command and sends it to the input of another command.

• ls -l creates a list of files.
• | pipes the list from ls into the command that follows.
• grep searches the files from ls for the string that follows.
• *.txt matches any file that ends with .txt.

Question 8

Compound commands with pipes typically follow this format:

Answer 8

program -options arguments | program -options | program -options | program -options

Question 9

What are some common programs users pipe to:(5)

Answer 9

• | head prints only the first 10 lines of output.
• | tail prints only the last 10 lines of output.
• | sort sorts the output alphabetically.
• | sed searches and replaces parts of the output.
• | awk display only specified parts of the output.

Question 10

Explain this command:

cat /etc/passwd | grep sysadmin | awk -F ‘:’ ‘{print $6}’

Answer 10

• cat /etc/passwd dumps the contents of /etc/passwd to output.
• | pipes that output into the command that follows.
• grep sysadmin displays lines that contain sysadmin.
• | pipes that output into the command that follows.
• awk -F ':' '{print $6}' prints only the sixth field of the line.
• awk usually looks for a space to use as a field separator, but in this case we want it to separate the line by a colon, because /etc/passwd uses colons to separate its fields.

Question 11

In addition to |, you can use ____ to run a series of commands back to back.

Answer 11

You can also use a ; to run a series of commands back to back.

Question 12

Rather than typing the following, how would you use ; :

bash
 $ mkdir dir
 $ cd dir
 $ touch file
 $ ls -l
 -rw-r--r-- 1 user user 0 Sep 4 15:33 file

Answer 12

mkdir dir; cd dir; touch file; ls -l

Each command will happen in succession.
- First, the mkdir command, then cd, touch, and finally ls.

Question 13

Compound commands using ; typically follow this format:

Answer 13

program -options arguments ; program -options arguments ; program -options arguments ; program -options arguments

For example: `mkdir dir; cd dir; touch file; ls -l

• The output would be:

bash
 $ mkdir dir; cd dir; touch file; ls -l
 -rw-r--r-- 1 user user 0 Sep 4 15:33 file

Question 14

If you only want to run the second command if the fist command was successful, what operator would you use?

Answer 14

&&

Question 15

Explain this command:

mkdir dir && cd dir && touch file && ls -l

Answer 15

cd would only run if mkdir were successful, touch would only run if cd were successful and ls would only run if touch were successful.

Question 16

Compound commands using && typically follow this format:

Answer 16

program -options arguments && program -options arguments && program -options arguments && program -options arguments

Question 17

Explain these operators:

1. >
2. >>
3. |
4. ;
5. &&

Answer 17

1. > to create files with the output of a command.
2. >> to append the output of a command to a file. Creates a file if the file does not exist.
3. | pipes the output of one command into another command.
4. -; to chain commands together in succession.
5. && to chain commands together. The second command runs only if the first command was successful.

Question 18

Compound commands are useful but _do_ require a lot of typing. If you use a compound command often, it might be nice to save it somewhere so you can easily reference it. You can do this by creating an ________.

Answer 18

Alias

Question 19

An _______ is a shorthand or custom command that you can define, which will launch any command or compound command, including arguments and redirection.

Answer 19

An **alias** is a shorthand or custom command that you can define, which will launch any command or compound command, including arguments and redirection.

Question 20

In computer programing, a ________ is a location that stores some kind of data.

Answer 20

In computer programing, a variable is a location that stores some kind of data.

• We can think of it as a box that holds something so you can refer to it later.
• If you no longer need what is in the box, you can overwrite its contents with new contents.

Question 21

Bash has a number of built-in variables called _______. They are also known as _______.

Answer 21

Bash has a number of built-in variables called **environment variables**. They are also known as **shell variables**.

Question 22

True or False:

Built-in variables are always defined with all lower case letters.

Answer 22

False

They are always defined with all upper case letters. For example, $PWD is an environment variable that returns the pwd command.

Question 23

What do these built-in variables do?

• echo "My name is $USER"
• echo "My home directory is $HOME"
• echo "The name of my computer is $HOSTNAME"
• echo "My type of computer is a $MACHTYPE"
• echo "My user ID is $UID":

Answer 23

• echo "My name is $USER": Provides the user name of the current user.
• echo "My home directory is $HOME": Provides the home folder of the current user.
• echo "The name of my computer is $HOSTNAME": Provides the name of the computer.
• echo "My type of computer is a $MACHTYPE": Provides the type of computer
• echo "My user ID is $UID": Provides the UID of the current user.

Question 24

________ in bash refers to any time something on the command line expands or morphs into something else.

Answer 24

**Expansion** in bash refers to any time something on the command line expands or morphs into something else.

Question 25

What part of the following command \_expands\_? file $(find / -iname \*.txt 2\>/dev/null) \> ~/Desktop/text\ files ; tail ~/Desktop/text\ files

Answer 25

In the command, the `find` command between the `$()` runs before any other part of the command. This `find` command \_expands\_ into a list of items that it found. The rest of the commands are acting on that list, not acting on the `find` command itself.

Question 26

True or False: Bash syntax uses the `$()` for command expansion.

Answer 26

True ## Footnote - You can put any amount of commands chained together inside these brackets. - Bash reads that chunk as whatever is returned from running the commands inside it. - Then, the rest of the commands on the line run. This is quite helpful when writing a script if we want one command to run before another command.

Question 27

True or False Bash script files often end in `.sh` to indicate they are a `shell script`. However, a script file will still run with any extension.

Answer 27

True ## Footnote Linux generally ignores file extensions. Instead, it looks at the contents of the file in order to determine how to use it. Therefore, you can create text files without any extension at all, but it is best practice to use the `.sh` file if you think other users may interact with your script.

Question 28

One of the most important environment variables on Linux operating systems is the _______ variable. This variable holds the colon-separated list of directories used to find commands that you enter.

Answer 28

One of the most important environment variables on Linux operating systems is the **PATH** variable. This variable holds the colon-separated list of directories used to find commands that you enter. The sole purpose of this environment variable is to hold a list of directories. For example, if PATH is set to /bin:/usr/sbin and you type echo, Linux looks for an executable program called echo in /bin and then in /usr/sbin.