Advanced Bash-Scripting Guide echo var2="\\\\\"" echo $var2 # " echo "$var2" # \\" echo
# But ... var2="\\\\"" is illegal. Why?
var3='\\\\' echo "$var3" # \\\\
# Strong quoting works, though.
# ************************************************************ #
# As the first example above shows, nesting quotes is permitted.
echo "$(echo '"')" # "
# ^ ^
# At times this comes in useful.
var1="Two bits" echo "\$var1 = "$var1"" # $var1 = Two bits
# ^ ^
# Or, as Chris Hiestand points out ...
if [[ "$(du "$My_File1")" -gt "$(du "$My_File2")" ]]
# ^ ^ ^ ^ ^ ^ ^ ^ then
...
fi
# ************************************************************ #
Single quotes (' ') operate similarly to double quotes, but do not permit referencing variables, since the special meaning of $ is turned off. Within single quotes, every special character except ' gets interpreted literally.
Consider single quotes ("full quoting") to be a stricter method of quoting than double quotes ("partial quoting").
Since even the escape character (\) gets a literal interpretation within single quotes, trying to enclose a single quote within single quotes will not yield the expected result.
echo "Why can't I write 's between single quotes" echo
# The roundabout method.
echo 'Why can'\''t I write '"'"'s between single quotes'
# |-------| |----------| |-----------------------|
# Three single-quoted strings, with escaped and quoted single quotes between.
# This example courtesy of Stéphane Chazelas.
5.2. Escaping
Escaping is a method of quoting single characters. The escape (\) preceding a character tells the shell to interpret that character literally.
Chapter 5. Quoting 43
Advanced Bash-Scripting Guide
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.
Chapter 5. Quoting 44
Advanced Bash-Scripting Guide 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 $escape"
# no visible output.
echo
Chapter 5. Quoting 45
exit 0
A more elaborate example:
Advanced Bash-Scripting Guide
Example 5-3. Detecting key-presses
#!/bin/bash
# Author: Sigurd Solaas, 20 Apr 2011
# Used in ABS Guide with permission.
# Requires version 4.2+ of Bash.
key="no value yet" while true; do
clear
echo "Bash Extra Keys Demo. Keys to try:"
echo
echo "* Insert, Delete, Home, End, Page_Up and Page_Down"
echo "* The four arrow keys"
echo "* Tab, enter, escape, and space key"
echo "* The letter and number keys, etc."
echo
echo " d = show date/time"
echo " q = quit"
echo "================================"
echo
# Convert the separate home-key to home-key_num_7:
if [ "$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
Chapter 5. Quoting 46
\"
\$
Advanced Bash-Scripting Guide
;;
$'\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 $7.98.
\\
Chapter 5. Quoting 47
Advanced Bash-Scripting Guide gives the backslash its literal meaning echo "\\" # Results in \
# Whereas . . .
echo "\" # Invokes secondary prompt from the command-line.
# In a script, gives an error message.
# However . . .
echo '\' # Results in \
The behavior of \ depends on whether it is escaped, strong-quoted, weak-quoted, or appearing within
command substitution or a here document.
# Simple escaping and quoting echo \z # z echo \\z # \z echo '\z' # \z echo '\\z' # \\z echo "\z" # \z echo "\\z" # \z
# Command substitution echo `echo \z` # z echo `echo \\z` # z echo `echo \\\z` # \z echo `echo \\\\z` # \z echo `echo \\\\\\z` # \z echo `echo \\\\\\\z` # \\z echo `echo "\z"` # \z echo `echo "\\z"` # \z
# Here document cat <<EOF
\z
EOF # \z cat <<EOF
\\z
EOF # \z
# These examples supplied by Stéphane Chazelas.
Elements of a string assigned to a variable may be escaped, but the escape character alone may not be assigned to a variable.
variable=\ echo "$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 "$variable"
#+ invalid variable assignment variable=\
23skidoo
Chapter 5. Quoting 48
Advanced Bash-Scripting Guide echo "$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
Chapter 5. Quoting 49
Advanced Bash-Scripting Guide 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.
Chapter 5. Quoting 50
