Escaping

Advanced Bash-Scripting Guide:
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# 5.2. Escaping

Escaping is a method of quoting single characters. The escape (\) preceding a character tells the shell to interpret that character literally.

| With certain commands and utilities, such as echo and sed, escaping a character may have the opposite effect - it can toggle on a special meaning for that character.

Special meanings of certain escaped characters

used with echo and sed

\n

  means newline

\r

  means return

\t

  means tab

\v

  means vertical tab

\b

  means backspace

\a

  means alert (beep or flash)

\0xx

  translates to the octal ASCII equivalent of `0nn`, where `nn` is a string of digits

  | The `$' ... '` quoted string-expansion construct is a mechanism that uses escaped octal or hex values to assign ASCII characters to variables, e.g., quote=$'\042'.

  Example 5-2. Escaped Characters

      #!/bin/bash
      # escaped.sh: escaped characters

      #############################################################
      ### First, let's show some basic escaped-character usage. ###
      #############################################################

      # Escaping a newline.
      # ------------------

      echo ""

      echo "This will print
      as two lines."
      # This will print
      # as two lines.

      echo "This will print \
      as one line."
      # This will print as one line.

      echo; echo

      echo "============="

      echo "\v\v\v\v"      # Prints \v\v\v\v literally.
      # Use the -e option with 'echo' to print escaped characters.
      echo "============="
      echo "VERTICAL TABS"
      echo -e "\v\v\v\v"   # Prints 4 vertical tabs.
      echo "=============="

      echo "QUOTATION MARK"
      echo -e "\042"       # Prints " (quote, octal ASCII character 42).
      echo "=============="

      # The $'\X' construct makes the -e option unnecessary.

      echo; echo "NEWLINE and (maybe) BEEP"
      echo $'\n'           # Newline.
      echo $'\a'           # Alert (beep).
                           # May only flash, not beep, depending on terminal.

      # We have seen $'\nnn" string expansion, and now . . .

      # =================================================================== #
      # Version 2 of Bash introduced the $'\nnn' string expansion construct.
      # =================================================================== #

      echo "Introducing the \$\' ... \' string-expansion construct . . . "
      echo ". . . featuring more quotation marks."

      echo $'\t \042 \t'   # Quote (") framed by tabs.
      # Note that  '\nnn' is an octal value.

      # It also works with hexadecimal values, in an $'\xhhh' construct.
      echo $'\t \x22 \t'  # Quote (") framed by tabs.
      # Thank you, Greg Keraunen, for pointing this out.
      # Earlier Bash versions allowed '\x022'.

      echo

      # Assigning ASCII characters to a variable.
      # ----------------------------------------
      quote=$'\042'        # " assigned to a variable.
      echo "$quote Quoted string $quote and this lies outside the quotes."

      echo

      # Concatenating ASCII chars in a variable.
      triple_underline=$'\137\137\137'  # 137 is octal ASCII code for '_'.
      echo "$triple_underline UNDERLINE $triple_underline"

      echo

      ABC=$'\101\102\103\010'           # 101, 102, 103 are octal A, B, C.
      echo $ABC

      echo

      escape=$'\033'                    # 033 is octal for escape.
      echo "\"escape\" echoes as $escape"
      #                                   no visible output.

      echo

      exit 0

  A more elaborate example:

  Example 5-3. Detecting key-presses

      #!/bin/bash
      # Author: Sigurd Solaas, 20 Apr 2011
      # Used in ABS Guide with permission.
      # Requires version 4.2+ of Bash.

      key="no value yet"
      while true; do
        clear
        echo "Bash Extra Keys Demo. Keys to try:"
        echo
        echo "* Insert, Delete, Home, End, Page_Up and Page_Down"
        echo "* The four arrow keys"
        echo "* Tab, enter, escape, and space key"
        echo "* The letter and number keys, etc."
        echo
        echo "    d = show date/time"
        echo "    q = quit"
        echo "================================"
        echo

       # Convert the separate home-key to home-key_num_7:
       if [ "$key" = $'\x1b\x4f\x48' ]; then
        key=$'\x1b\x5b\x31\x7e'
        #   Quoted string-expansion construct.
       fi

       # Convert the separate end-key to end-key_num_1.
       if [ "$key" = $'\x1b\x4f\x46' ]; then
        key=$'\x1b\x5b\x34\x7e'
       fi

       case "$key" in
        $'\x1b\x5b\x32\x7e')  # Insert
         echo Insert Key
        ;;
        $'\x1b\x5b\x33\x7e')  # Delete
         echo Delete Key
        ;;
        $'\x1b\x5b\x31\x7e')  # Home_key_num_7
         echo Home Key
        ;;
        $'\x1b\x5b\x34\x7e')  # End_key_num_1
         echo End Key
        ;;
        $'\x1b\x5b\x35\x7e')  # Page_Up
         echo Page_Up
        ;;
        $'\x1b\x5b\x36\x7e')  # Page_Down
         echo Page_Down
        ;;
        $'\x1b\x5b\x41')  # Up_arrow
         echo Up arrow
        ;;
        $'\x1b\x5b\x42')  # Down_arrow
         echo Down arrow
        ;;
        $'\x1b\x5b\x43')  # Right_arrow
         echo Right arrow
        ;;
        $'\x1b\x5b\x44')  # Left_arrow
         echo Left arrow
        ;;
        $'\x09')  # Tab
         echo Tab Key
        ;;
        $'\x0a')  # Enter
         echo Enter Key
        ;;
        $'\x1b')  # Escape
         echo Escape Key
        ;;
        $'\x20')  # Space
         echo Space Key
        ;;
        d)
         date
        ;;
        q)
        echo Time to quit...
        echo
        exit 0
        ;;
        *)
         echo You pressed: \'"$key"\'
        ;;
       esac

       echo
       echo "================================"

       unset K1 K2 K3
       read -s -N1 -p "Press a key: "
       K1="$REPLY"
       read -s -N2 -t 0.001
       K2="$REPLY"
       read -s -N1 -t 0.001
       K3="$REPLY"
       key="$K1$K2$K3"

      done

      exit $?

  See also Example 37-1.

\"

  gives the quote its literal meaning

      echo "Hello"                     # Hello
      echo "\"Hello\" ... he said."    # "Hello" ... he said.

\$

  gives the dollar sign its literal meaning (variable name following \$ will not be referenced)

      echo "\$variable01"           # $variable01
      echo "The book cost \$7.98."  # The book cost $7.98.

\\

  gives the backslash its literal meaning

      echo "\\"  # Results in \

      # Whereas . . .

      echo "\"   # Invokes secondary prompt from the command-line.
                 # In a script, gives an error message.

      # However . . .

      echo '\'   # Results in \

| The behavior of \ depends on whether it is escaped, strong-quoted, weak-quoted, or appearing within command substitution or a here document.
                          #  Simple escaping and quoting
    echo \z               #  z
    echo \\z              # \z
    echo '\z'             # \z
    echo '\\z'            # \\z
    echo "\z"             # \z
    echo "\\z"            # \z

                          #  Command substitution
    echo `echo \z`        #  z
    echo `echo \\z`       #  z
    echo `echo \\\z`      # \z
    echo `echo \\\\z`     # \z
    echo `echo \\\\\\z`   # \z
    echo `echo \\\\\\\z`  # \\z
    echo `echo "\z"`      # \z
    echo `echo "\\z"`     # \z

                          # Here document
    cat <<EOF
    \z
    EOF                   # \z

    cat <<EOF
    \\z
    EOF                   # \z

    # These examples supplied by Stéphane Chazelas.
Elements of a string assigned to a variable may be escaped, but the escape character alone may not be assigned to a variable.
    variable=\
    echo "$variable"
    # Will not work - gives an error message:
    # test.sh: : command not found
    # A "naked" escape cannot safely be assigned to a variable.
    #
    #  What actually happens here is that the "\" escapes the newline and
    #+ the effect is        variable=echo "$variable"
    #+                      invalid variable assignment

    variable=\
    23skidoo
    echo "$variable"        #  23skidoo
                            #  This works, since the second line
                            #+ is a valid variable assignment.

    variable=\
    #        \^    escape followed by space
    echo "$variable"        # space

    variable=\\
    echo "$variable"        # \

    variable=\\\
    echo "$variable"
    # Will not work - gives an error message:
    # test.sh: \: command not found
    #
    #  First escape escapes second one, but the third one is left "naked",
    #+ with same result as first instance, above.

    variable=\\\\
    echo "$variable"        # \\
                            # Second and fourth escapes escaped.
                            # This is o.k.

Escaping a space can prevent word splitting in a command's argument list.

    file_list="/bin/cat /bin/gzip /bin/more /usr/bin/less /usr/bin/emacs-20.7"
    # List of files as argument(s) to a command.

    # Add two files to the list, and list all.
    ls -l /usr/X11R6/bin/xsetroot /sbin/dump $file_list

    echo "-------------------------------------------------------------------------"

    # What happens if we escape a couple of spaces?
    ls -l /usr/X11R6/bin/xsetroot\ /sbin/dump\ $file_list
    # Error: the first three files concatenated into a single argument to 'ls -l'
    #        because the two escaped spaces prevent argument (word) splitting.

The escape also provides a means of writing a multi-line command. Normally, each separate line constitutes a different command, but an escape at the end of a line escapes the newline character, and the command sequence continues on to the next line.

    (cd /source/directory && tar cf - . ) | \
    (cd /dest/directory && tar xpvf -)
    # Repeating Alan Cox's directory tree copy command,
    # but split into two lines for increased legibility.

    # As an alternative:
    tar cf - -C /source/directory . |
    tar xpvf - -C /dest/directory
    # See note below.
    # (Thanks, Stéphane Chazelas.)

| If a script line ends with a |, a pipe character, then a \, an escape, is not strictly necessary. It is, however, good programming practice to always escape the end of a line of code that continues to the following line.

    echo "foo
    bar"
    #foo
    #bar

    echo

    echo 'foo
    bar'    # No difference yet.
    #foo
    #bar

    echo

    echo foo\
    bar     # Newline escaped.
    #foobar

    echo

    echo "foo\
    bar"     # Same here, as \ still interpreted as escape within weak quotes.
    #foobar

    echo

    echo 'foo\
    bar'     # Escape character \ taken literally because of strong quoting.
    #foo\
    #bar

    # Examples suggested by Stéphane Chazelas.

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