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 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

   1 #!/bin/bash
   2 # escaped.sh: escaped characters
   3 
   4 #############################################################
   5 ### First, let's show some basic escaped-character usage. ###
   6 #############################################################
   7 
   8 # Escaping a newline.
   9 # ------------------
  10 
  11 echo ""
  12 
  13 echo "This will print
  14 as two lines."
  15 # This will print
  16 # as two lines.
  17 
  18 echo "This will print \
  19 as one line."
  20 # This will print as one line.
  21 
  22 echo; echo
  23 
  24 echo "============="
  25 
  26 
  27 echo "\v\v\v\v"      # Prints \v\v\v\v literally.
  28 # Use the -e option with 'echo' to print escaped characters.
  29 echo "============="
  30 echo "VERTICAL TABS"
  31 echo -e "\v\v\v\v"   # Prints 4 vertical tabs.
  32 echo "=============="
  33 
  34 echo "QUOTATION MARK"
  35 echo -e "\042"       # Prints " (quote, octal ASCII character 42).
  36 echo "=============="
  37 
  38 
  39 
  40 # The $'\X' construct makes the -e option unnecessary.
  41 
  42 echo; echo "NEWLINE and (maybe) BEEP"
  43 echo $'\n'           # Newline.
  44 echo $'\a'           # Alert (beep).
  45                      # May only flash, not beep, depending on terminal.
  46 
  47 # We have seen $'\nnn" string expansion, and now . . .
  48 
  49 # =================================================================== #
  50 # Version 2 of Bash introduced the $'\nnn' string expansion construct.
  51 # =================================================================== #
  52 
  53 echo "Introducing the \$\' ... \' string-expansion construct . . . "
  54 echo ". . . featuring more quotation marks."
  55 
  56 echo $'\t \042 \t'   # Quote (") framed by tabs.
  57 # Note that  '\nnn' is an octal value.
  58 
  59 # It also works with hexadecimal values, in an $'\xhhh' construct.
  60 echo $'\t \x22 \t'  # Quote (") framed by tabs.
  61 # Thank you, Greg Keraunen, for pointing this out.
  62 # Earlier Bash versions allowed '\x022'.
  63 
  64 echo
  65 
  66 
  67 # Assigning ASCII characters to a variable.
  68 # ----------------------------------------
  69 quote=$'\042'        # " assigned to a variable.
  70 echo "$quote Quoted string $quote and this lies outside the quotes."
  71 
  72 echo
  73 
  74 # Concatenating ASCII chars in a variable.
  75 triple_underline=$'\137\137\137'  # 137 is octal ASCII code for '_'.
  76 echo "$triple_underline UNDERLINE $triple_underline"
  77 
  78 echo
  79 
  80 ABC=$'\101\102\103\010'           # 101, 102, 103 are octal A, B, C.
  81 echo $ABC
  82 
  83 echo
  84 
  85 escape=$'\033'                    # 033 is octal for escape.
  86 echo "\"escape\" echoes as $escape"
  87 #                                   no visible output.
  88 
  89 echo
  90 
  91 exit 0

A more elaborate example:

Example 5-3. Detecting key-presses

   1 #!/bin/bash
   2 # Author: Sigurd Solaas, 20 Apr 2011
   3 # Used in ABS Guide with permission.
   4 # Requires version 4.2+ of Bash.
   5 
   6 key="no value yet"
   7 while true; do
   8   clear
   9   echo "Bash Extra Keys Demo. Keys to try:"
  10   echo
  11   echo "* Insert, Delete, Home, End, Page_Up and Page_Down"
  12   echo "* The four arrow keys"
  13   echo "* Tab, enter, escape, and space key"
  14   echo "* The letter and number keys, etc."
  15   echo
  16   echo "    d = show date/time"
  17   echo "    q = quit"
  18   echo "================================"
  19   echo
  20 
  21  # Convert the separate home-key to home-key_num_7:
  22  if [ "$key" = $'\x1b\x4f\x48' ]; then
  23   key=$'\x1b\x5b\x31\x7e'
  24   #   Quoted string-expansion construct. 
  25  fi
  26 
  27  # Convert the separate end-key to end-key_num_1.
  28  if [ "$key" = $'\x1b\x4f\x46' ]; then
  29   key=$'\x1b\x5b\x34\x7e'
  30  fi
  31 
  32  case "$key" in
  33   $'\x1b\x5b\x32\x7e')  # Insert
  34    echo Insert Key
  35   ;;
  36   $'\x1b\x5b\x33\x7e')  # Delete
  37    echo Delete Key
  38   ;;
  39   $'\x1b\x5b\x31\x7e')  # Home_key_num_7
  40    echo Home Key
  41   ;;
  42   $'\x1b\x5b\x34\x7e')  # End_key_num_1
  43    echo End Key
  44   ;;
  45   $'\x1b\x5b\x35\x7e')  # Page_Up
  46    echo Page_Up
  47   ;;
  48   $'\x1b\x5b\x36\x7e')  # Page_Down
  49    echo Page_Down
  50   ;;
  51   $'\x1b\x5b\x41')  # Up_arrow
  52    echo Up arrow
  53   ;;
  54   $'\x1b\x5b\x42')  # Down_arrow
  55    echo Down arrow
  56   ;;
  57   $'\x1b\x5b\x43')  # Right_arrow
  58    echo Right arrow
  59   ;;
  60   $'\x1b\x5b\x44')  # Left_arrow
  61    echo Left arrow
  62   ;;
  63   $'\x09')  # Tab
  64    echo Tab Key
  65   ;;
  66   $'\x0a')  # Enter
  67    echo Enter Key
  68   ;;
  69   $'\x1b')  # Escape
  70    echo Escape Key
  71   ;;
  72   $'\x20')  # Space
  73    echo Space Key
  74   ;;
  75   d)
  76    date
  77   ;;
  78   q)
  79   echo Time to quit...
  80   echo
  81   exit 0
  82   ;;
  83   *)
  84    echo You pressed: \'"$key"\'
  85   ;;
  86  esac
  87 
  88  echo
  89  echo "================================"
  90 
  91  unset K1 K2 K3
  92  read -s -N1 -p "Press a key: "
  93  K1="$REPLY"
  94  read -s -N2 -t 0.001
  95  K2="$REPLY"
  96  read -s -N1 -t 0.001
  97  K3="$REPLY"
  98  key="$K1$K2$K3"
  99 
 100 done
 101 
 102 exit $?

See also Example 37-1.

\"

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" # $variable01 2 echo "The book cost \$7.98." # The book cost $7.98.

\\

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. 7 8 # However . . . 9 10 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.
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.