Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting
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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 0xx

Example 5-2. Escaped Characters

   1 #!/bin/bash
   2 # escaped.sh: escaped characters
   3 
   4 echo; echo
   5 
   6 # Escaping a newline.
   7 # ------------------
   8 
   9 echo ""
  10 
  11 echo "This will print
  12 as two lines."
  13 # This will print
  14 # as two lines.
  15 
  16 echo "This will print \
  17 as one line."
  18 # This will print as one line.
  19 
  20 echo; echo
  21 
  22 echo "============="
  23 
  24 
  25 echo "\v\v\v\v"      # Prints \v\v\v\v literally.
  26 # Use the -e option with 'echo' to print escaped characters.
  27 echo "============="
  28 echo "VERTICAL TABS"
  29 echo -e "\v\v\v\v"   # Prints 4 vertical tabs.
  30 echo "=============="
  31 
  32 echo "QUOTATION MARK"
  33 echo -e "\042"       # Prints " (quote, octal ASCII character 42).
  34 echo "=============="
  35 
  36 # The $'\X' construct makes the -e option unnecessary.
  37 echo; echo "NEWLINE AND BEEP"
  38 echo $'\n'           # Newline.
  39 echo $'\a'           # Alert (beep).
  40 
  41 echo "==============="
  42 echo "QUOTATION MARKS"
  43 # Version 2 and later of Bash permits using the $'\nnn' construct.
  44 # Note that in this case, '\nnn' is an octal value.
  45 echo $'\t \042 \t'   # Quote (") framed by tabs.
  46 
  47 # It also works with hexadecimal values, in an $'\xhhh' construct.
  48 echo $'\t \x22 \t'  # Quote (") framed by tabs.
  49 # Thank you, Greg Keraunen, for pointing this out.
  50 # Earlier Bash versions allowed '\x022'.
  51 echo "==============="
  52 echo
  53 
  54 
  55 
  56 
  57 
  58 # Assigning ASCII characters to a variable.
  59 # ----------------------------------------
  60 quote=$'\042'        # " assigned to a variable.
  61 echo "$quote This is a quoted string, $quote and this lies outside the quotes."
  62 
  63 echo
  64 
  65 # Concatenating ASCII chars in a variable.
  66 triple_underline=$'\137\137\137'  # 137 is octal ASCII code for '_'.
  67 echo "$triple_underline UNDERLINE $triple_underline"
  68 
  69 echo
  70 
  71 ABC=$'\101\102\103\010'           # 101, 102, 103 are octal A, B, C.
  72 echo $ABC
  73 
  74 echo; echo
  75 
  76 escape=$'\033'                    # 033 is octal for escape.
  77 echo "\"escape\" echoes as $escape"
  78 #                                   no visible output.
  79 
  80 echo; echo
  81 
  82 exit 0

See Example 34-1 for another example of the $' ' string expansion construct.

\"

gives the quote its literal meaning

1 echo "Hello" # Hello 2 echo "\"Hello\", he said." # "Hello", he said.

\$

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

1 echo "\$variable01" # results in $variable01

\\

gives the backslash its literal meaning

1 echo "\\" # Results in \ 2 3 # Whereas . . . 4 5 echo "\" # Invokes secondary prompt from the command line. 6 # In a script, gives an error message.

The behavior of \ depends on whether it is itself escaped, quoted, or appearing within command substitution or a here document.

1 # Simple escaping and quoting 2 echo \z # z 3 echo \\z # \z 4 echo '\z' # \z 5 echo '\\z' # \\z 6 echo "\z" # \z 7 echo "\\z" # \z 8 9 # Command substitution 10 echo `echo \z` # z 11 echo `echo \\z` # z 12 echo `echo \\\z` # \z 13 echo `echo \\\\z` # \z 14 echo `echo \\\\\\z` # \z 15 echo `echo \\\\\\\z` # \\z 16 echo `echo "\z"` # \z 17 echo `echo "\\z"` # \z 18 19 # Here document 20 cat <<EOF 21 \z 22 EOF # \z 23 24 cat <<EOF 25 \\z 26 EOF # \z 27 28 # 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.

1 variable=\ 2 echo "$variable" 3 # Will not work - gives an error message: 4 # test.sh: : command not found 5 # A "naked" escape cannot safely be assigned to a variable. 6 # 7 # What actually happens here is that the "\" escapes the newline and 8 #+ the effect is variable=echo "$variable" 9 #+ invalid variable assignment 10 11 variable=\ 12 23skidoo 13 echo "$variable" # 23skidoo 14 # This works, since the second line 15 #+ is a valid variable assignment. 16 17 variable=\ 18 # \ escape followed by space 19 echo "$variable" # space 20 21 variable=\\ 22 echo "$variable" # \ 23 24 variable=\\\ 25 echo "$variable" 26 # Will not work - gives an error message: 27 # test.sh: \: command not found 28 # 29 # First escape escapes second one, but the third one is left "naked", 30 #+ with same result as first instance, above. 31 32 variable=\\\\ 33 echo "$variable" # \\ 34 # Second and fourth escapes escaped. 35 # This is o.k.

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

1 file_list="/bin/cat /bin/gzip /bin/more /usr/bin/less /usr/bin/emacs-20.7" 2 # List of files as argument(s) to a command. 3 4 # Add two files to the list, and list all. 5 ls -l /usr/X11R6/bin/xsetroot /sbin/dump $file_list 6 7 echo "-------------------------------------------------------------------------" 8 9 # What happens if we escape a couple of spaces? 10 ls -l /usr/X11R6/bin/xsetroot\ /sbin/dump\ $file_list 11 # Error: the first three files concatenated into a single argument to 'ls -l' 12 # 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.

1 (cd /source/directory && tar cf - . ) | \ 2 (cd /dest/directory && tar xpvf -) 3 # Repeating Alan Cox's directory tree copy command, 4 # but split into two lines for increased legibility. 5 6 # As an alternative: 7 tar cf - -C /source/directory . | 8 tar xpvf - -C /dest/directory 9 # See note below. 10 # (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.

1 echo "foo 2 bar" 3 #foo 4 #bar 5 6 echo 7 8 echo 'foo 9 bar' # No difference yet. 10 #foo 11 #bar 12 13 echo 14 15 echo foo\ 16 bar # Newline escaped. 17 #foobar 18 19 echo 20 21 echo "foo\ 22 bar" # Same here, as \ still interpreted as escape within weak quotes. 23 #foobar 24 25 echo 26 27 echo 'foo\ 28 bar' # Escape character \ taken literally because of strong quoting. 29 #foo\ 30 #bar 31 32 # Examples suggested by Stéphane Chazelas.

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