Page Comparison

We've looked at some ways of viewing text, so now we'll address how to write it.

echo - the bash print function

Other programming languages usually supply a print statement or function that can direct text to files a file or to standard output.

In bash, the print utility directs text to actual printers, so there is a different method of text output: the echo command. Let's give it a try:

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echo hello     # displays "hello" on the next terminal line, then the prompt on a new line
echo -n hello  # displays "hello" on the next terminal line followed by the prompt (no newline)

Exercise 3-1

What is the difference in character count when you echo hello with and without the -n option?

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The reason for this difference is that by default, echo actually reads then outputs all the characters on the line, including the trailing linefeed. To understand this better, and to understand what is meant by "interpretation of backslash escapes", we need to first look at how the shell evaluates other metacharacters, and how that is affected by quoting in the shell.

Environment variables

Environment variables are just like variables in a programming language (in fact bash is a complete programming language): they are "names" that hold a value assigned to them. As with all programming language variables, they have two operations:

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There are a number of pre-defined environment variables in the shell, such as USER (your account name) and PATH (more on PATH later). The env command will list them all along with their values.

Exercise 3-2

Output a string that includes your account name and your Unix group name using environment variables.

env | grep -i group  # is there an environment variable with "group" in its name?

echo The Unix group for $USER is $MY_GROUP

Quoting in the shell

When the shell processes a command line, it first parses the text into tokens ("words"), which are groups of characters separated by whitespace (one or more space characters). Quoting affects how this parsing happens, including how metacharacters are treated and how text is grouped.

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1. single quoting (e.g. 'some text') – this serves two purposes
   • it groups together all text inside the quotes into a single token
   • it tells the shell not to "look inside" the quotes to perform anyevaluationsevaluation
     
     • all metacharacters inside the single quotes are ignored
     • in particular, any environment variables in single-quoted text are not evaluated
2. double quoting (e.g. "some text") – also serves two purposes
   • it groups together all text inside the quotes into a single token
   • it allows environment variable evaluation, but inhibits some metacharcters
     • e.g. asterisk ( * ) pathname globbing (more on globbing later...)
     • and some other metacharacters
       
   • double quoting also preserves any special characters in the text
     • e.g. newlines (\n) or Tabs (\t)
3. backtick quoting (e.g. `date`)
   
   • evaluates the expression inside the backtick marks ( ` )
   • the standard output of the expression replaces the text inside the backtick marks ( ` )

Note that the quote characters themselves ( '  "  ` ) are metacharacters that tell the shell to "start a quoting process" then "end a quoting process" when the matching quote is found. Since they are part of the processing, the enclosing quotes are not included in the output.

Let's look at examples of these.

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Let's look at examples of these.

Single and double quotes

The first rule of quoting is: always enclose a command argument in quotes if it contains spaces so that the command sees the quoted text as one item. See the difference between:

echo 'Hello world!'     # the argument "Hello world!" 

To see more on how quoting

To see more on how quoting affects text grouping, we'll use quotes to define some multi-word environment variables.

See the difference between:

foo="My name is Anna"; echo $foo
foo='My name is Anna'; echo $foo  

The above two expressions produce the same output because the assigned text does not contain any special metacharacters.

But these two expressions are different:

foo="My USER name is $USER"; echo $foo   # The text "$USER" is evaluated and its value substituted
foo='My USER name is $USER'; echo $foo   # The text "$USER" is left as-is

Here the single quotes tell the shell to treat the quoted text as a literal, and not to look inside it for metacharacter processing.

So far so good. But what if you want text to include quotes? For example, if you want to output this text:

The value of variable 'FOO' is "$FOO"

Two common approaches:

• Use the backslash ( \ ) character that escapes the following character
  • escaping means treat the next character as a literal, even if it is a special metacharacter
• Use combinations of single and double quotes.

Examples

FOO="Hello world!"
echo "The value of variable 'FOO' is \"$FOO\""   # Escape the double quotes inside double quotes
echo "The value of variable 'FOO' is"' "$FOO"'   # Single-quoted text after double-quoted text
echo 'The value of variable 'FOO' is "$FOO"'     # Two single-quoted strings along with unquoted text FOO

Exercise 3-3

How would you output this text: The backslash character \ is used for escaping

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A couple of possibilities: 

echo 'The backslash character \ is used for escaping'  # Single quotes inhibit metacharacter processing
echo "The backslash character \\ is used for escaping" # Escape the escape character

Multi-line text

If you want to output multi-line text, you can:

• Start the text with a single or double quote
  • press Enter when you want to start a new line
  • keep entering text and Enter
  • until you're satisfied
  • enter the matching single or double quote then Enter
• Use echo -e to "enable interpretation of backslash escapes"
  
  • Now we know what that means!
  • Note that backslash escapes include some that represent non-printable characters
    • e.g. newline/linefeed ( \n ), and tab ( \t )

Exercise 3-4

How would you output this text:

My
name is
Anna

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A couple of possibilities: 

echo 'My
name is
Anna'

echo -e "My\nname\nis\nAnna"

Backtick evaluation quoting

backtick ( ` ) evaluation quoting is one of the underappreciated wonders of Unix.

The shell:

• evaluates the expression/command inside the backtick marks ( ` )
• the standard output of the expression replaces the text inside the backtick marks ( ` )

Examples, using the date function that just writes the current date and time to standard output, which appears on your Terminal.

date          # Calling the date command just displays date/time information
echo date     # Here "date" is treated as a literal word, and written to output
echo `date`   # The date command is evaluated and its output replaces the command'Hello world'   # correct - defines variable "foo" to have value "Hello world"
foo=Hello world     # error - no command called "world"

The 2nd expression above, without the quotes, produces an error. What's going on? The shell parses the input into two tokens: "foo=Hello" and "world". It assigns the value "Hello" to the variable foo, then tries to execute world, which it thinks is a command.

As for the difference between single quotes and double quotes, these two expressions produce the same output because the assigned text does not contain any special metacharacters:

foo="My name is Anna"; echo $foo
foo='My name is Anna'; echo $foo  

But these two expressions are different:

foo="My USER name is $USER"; echo $foo   # The text "$USER" is evaluated and its value substituted
foo='My USER name is $USER'; echo $foo   # The text "$USER" is left as-is

Here the single quotes tell the shell to treat the quoted text as a literal, and not to look inside it for metacharacter processing.

So far so good. But what if you want text to include quotes? For example, if you want to output this text:

The value of variable 'FOO' is "$FOO"

Two common approaches:

• Use the backslash ( \ ) character that escapes the following character
  • escaping means treat the next character as a literal, even if it is a special metacharacter
• Use combinations of single and double quotes.

Examples

FOO="Hello world!"
echo "The value of variable 'FOO' is \"$FOO\""   # Escape the double quotes inside double quotes
echo "The value of variable 'FOO' is "'"'$FOO'"' # Single-quoted text after double-quoted text

Note that the quote characters themselves ( '  "  ` ) are metacharacters that tell the shell to "start a quoting process" then "end a quoting process" when the matching quote is found. Since they are part of the processing, the enclosing quotes are not included in the output.

Exercise 3-3

How would you output this text: The backslash character \ is used for escaping

echo 'The backslash character \ is used for escaping'  # Single quotes inhibit metacharacter processing
echo "The backslash character \\ is used for escaping" # Escape the escape character

Multi-line text

If you want to output multi-line text, you can:

• Start the text with a single or double quote
  • press Enter when you want to start a new line
  • keep entering text and Enter
  • until you're satisfied
  • enter the matching single or double quote then Enter
• Use echo -e to "enable interpretation of backslash escapes"
  
  • Now we know what that means!
  • Note that backslash escapes include some that represent non-printable characters
    • e.g. newline/linefeed ( \n ), and tab ( \t )

Exercise 3-4

How would you output this text:

My
name is
Anna

echo 'My
name is
Anna'

echo -e "My\nname\nis\nAnna"

Backtick evaluation quoting

backtick ( ` ) evaluation quoting is one of the underappreciated wonders of Unix.

The shell:

• evaluates the expression/command inside the backtick marks ( ` )
• the standard output of the expression replaces the text inside the backtick marks ( ` )

Examples, using the date function that just writes the current date and time to standard output, which appears on your Terminal.

date          # Calling the date command just displays date/time information
echo date     # Here "date" is treated as a literal word, and written to output
echo `date`   # The date command is evaluated and its output replaces the command

# Assign a string including today's date to variable "today"
today="Today is: `date`"; echo $today  

The slightly odd syntax $(<some command>), is equivalent to `<some command>`, and can be easier to read when the command to be evaluated is complex.

echo "Today is: `date`"
echo "Today is $(date)"

Exercise 3-5

How would you output this text using a command to calculate the number of lines: The haiku.txt file has 11 lines

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echo "The haiku.txt file has `cat haiku.txt | wc -l` lines"

Redirection

So far text we've been working with has been output to standard output, which I keep reminding you is mapped to your Terminal. But you can redirect text elsewhere.

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You can also specify the tee -a option to append the input text to the file you specify.a option to append the input text to the file you specify.

Note that the > redirection metacharacter sends its output to a file, not to another program's standard input stream as with the | pipe metacharacter. (There are some cases where redirection involves something other than a file, but that's a topic for the Advanced Bash scripting class.)

The standard error stream

So what's this standard error stream? Recall our discussion of Command input errors? Well, error information is written to standard error, not to standard output!

It is easy to not notice the difference between standard output and standard error when you're in an interactive Terminal session – because both outputs are sent to the Terminal. But they are separate streams, with different meanings.

When executing commands you will want to manipulate standard output and standard error appropriately – especially for 3rd party programs.

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ls haiku.txt xxx.txt 1> stdoutout.txt 2>stderr2>err.txt
cat stdoutout.txt   # Displays "haiku.txt"
cat stderrerr.txt   # Displays "ls: cannot access 'xxx.txt': No such file or directory"

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One final redirection trick. There is a special Linux file called /dev/null that serves as a "global trash can". That is – it just throws away anything you write to it. So you can direct standard output and/or standard error to /dev/null to ignore it completely.

Exercise 3-6

Show the difference between  standard output and standard error by redirecting standard error to /dev/null.

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ls haiku.txt xxx.txt 1> /dev/null

Editing text

We've now covered viewing existing file text and writing new text to a file. But what if you want to edit/change text in an existing file?

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1. Use a command-line program that lets you enter/edit text in a Terminal window (e.g. nano, vi/vim, emacs)
   1. nano is extremely simple and is a good choice as a first local text editor
      • warning: nano has a tendency to break long single lines into multiple lines
   2. vi and emacs are extremely powerful but also quite complex
      • emacs reference sheet: https://www.gnu.org/software/emacs/refcards/pdf/refcard.pdf
      • vi reference sheet: http://www.atmos.albany.edu/daes/atmclasses/atm350/vi_cheat_sheet.pdf
2. Use a text editor or IDE (Integrated Development Environment) program that runs on your local computer but has an SFTP (secure FTP) interface that lets you connect to a remote computer
   
   • E.g.,Komodo IDE (Windows & Mac) or Notepad++ (Windows). Both are no-cost.
     
   • Once you connect to the remote host, you can navigate its directory structure and edit files.
   • When you open a file, its contents are brought over the network into the text editor's edit window, then saved back when you save the file.
3. Use software or protocols that allow you to "mount" remote server directories
   
   • Once mounted, the remote storage appears as a local volume/drive.
     • Then, you can use any text editor or IDE on your local computer to open/edit/save remote files.
   • Software programs that can mount remote data include ExpanDrive for Windows or Mac (costs $$, but has a free trial), TextWrangler for Mac.
   • Remote file system protocols include Samba (Windows, Mac) and NFS (Linux)

Knowing the basics of at least one Linux command-line text editor is useful for creating/editing small files, and we'll explore nano in this class. For editing larger files, you may find options #2 or #3 more useful.

nano

nano is a very simple editor available on most Linux systems. If you are able to ssh into a remote system, you can use nano there.

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Navigation and operations in nano are similar to those we discussed in Command line editing.

You can just type in text, and navigate around using arrow keys (up/down/left/right). A couple of other navigation shortcuts:

• Ctrl-a - go to start of line
• Ctrl-e - go to end of line
• Ctrl-v - go forward one page
• Arrow keys are also modified by Ctrl- (Windows) or Option- (Mac)
  
  • Ctrl-right-arrow (Windows) or Option-right-arrow (Mac) will skip by "word" forward
  • Ctrl-left-arrow (Windows) or Option-left-arrow (Mac) will skip by "word" backward

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• To delete text after the cursor, use:
  
  • Delete key on Windows
  • Function-Delete keys on Macintosh
• To delete text before the cursor, use:
  
  • Backspace key on Windows
  • Delete key on Macintosh
• Use Ctrl-k (kill) to delete everything on the line
  • This is different from Ctrl-k on the command line where it deletes everything after the cursor

Once you're satisfied with your edits:

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These and other important nano operations are displayed in a menu at the bottom of the Terminal window. Note that the ^ character means Ctrl- in this menu.

Edit your login script

Whenever you login to a remote server, a login script located in your Home directory is executed. This file, usually your ~/.profile and/or ~/.bashrc file, has expressions that customize your shell environment. These customizations are temporary – they are in effect only during your login sessions, which is why they have to be re-established every time you login.

On our system, ~/.profile file is your login script. Use more ~/.profile to look at it.

Exercise 3-7

Use nano to remove the pound sign ( # ) comment character from one of the two lines starting with #export LS_COLORS

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