Special meanings of certainescaped characters

used with echo andsed

means newline

means return

means tab

v

means vertical tab



means backspace

a

means alert (beep or flash)

xx

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

The $' ... ' quoted string-expansion construct is a mechanism that uses escaped octal or hex values to assign ASCII characters to variables, e.g., quote=$'42'.

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 printas two lines." # This will print# as two lines.echo "This will print as one line." # This will print as one line.echo; echoecho "=============" echo "vvvv" # Prints vvvv literally.# Use the -e option with 'echo' to print escaped characters.echo "=============" echo "VERTICAL TABS" echo -e "vvvv" # Prints 4 vertical tabs.echo "==============" echo "QUOTATION MARK" echo -e "42" # Prints " (quote, octal ASCII character 42).echo "==============" # The $'X' construct makes the -e option unnecessary.echo; echo "NEWLINE and (maybe) BEEP" echo $'' # Newline.echo $'a' # Alert (beep). # May only flash, not beep, depending on terminal.# We have seen $'nn" string expansion, and now . . .# =================================================================== ## Version 2 of Bash introduced the $'nn' string expansion construct.# =================================================================== #echo "Introducing the $' ... ' string-expansion construct . . . " echo ". . . featuring more quotation marks." echo $' 42 ' # Quote (") framed by tabs.# Note that 'nn' is an octal value.# It also works with hexadecimal values, in an $'xhhh' construct.echo $' x22 ' # Quote (") framed by tabs.# Thank you, Greg Keraunen, for pointing this out.# Earlier Bash versions allowed 'x022'.echo# Assigning ASCII characters to a variable.# ----------------------------------------quote=$'42' # " assigned to a variable.echo "$quote Quoted string $quote and this lies outside the quotes." echo# Concatenating ASCII chars in a variable.triple_underline=$'137137137' # 137 is octal ASCII code for '_'.echo "$triple_underline UNDERLINE $triple_underline" echoABC=$'10110210310' # 101, 102, 103 are octal A, B, C.echo $ABCechoescape=$'33' # 033 is octal for escape.echo ""escape" echoes as $escape" # no visible output.echoexit 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" = $'x1bx4fx48' ]; then key=$'x1bx5bx31x7e' # Quoted string-expansion construct. fi # Convert the separate end-key to end-key_num_1. if [ "$key" = $'x1bx4fx46' ]; then key=$'x1bx5bx34x7e' fi case "$key" in $'x1bx5bx32x7e') # Insert echo Insert Key ; $'x1bx5bx33x7e') # Delete echo Delete Key ; $'x1bx5bx31x7e') # Home_key_num_7 echo Home Key ; $'x1bx5bx34x7e') # End_key_num_1 echo End Key ; $'x1bx5bx35x7e') # Page_Up echo Page_Up ; $'x1bx5bx36x7e') # Page_Down echo Page_Down ; $'x1bx5bx41') # Up_arrow echo Up arrow ; $'x1bx5bx42') # Down_arrow echo Down arrow ; $'x1bx5bx43') # Right_arrow echo Right arrow ; $'x1bx5bx44') # 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" doneexit $?

See also Example 37-1.

"

gives the quote its literal meaning

echo "Hello" # Helloecho ""Hello" ... he said." # "Hello" ... he said.

$

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

echo "$variable01" # $variable01echo "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