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
#!/bin/bash
# escaped.sh: escaped characters
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 BEEP"
echo $'\n' # Newline.
echo $'\a' # Alert (beep).
echo "==============="
echo "QUOTATION MARKS"
# Version 2 and later of Bash permits using the $'\nnn' construct.
# Note that in this case, '\nnn' is an octal value.
echo $'\t \042 \t' # Quote (") framed by tabs.
# It also works with hexadecimal values, in an $'\xhhh' construct.
echo $'\t \x22 \t' # Quote (") framed by tabs.
# Thank you, Greg Keraunen, for pointing this out.
# Earlier Bash versions allowed '\x022'.
echo "==============="
echo
# Assigning ASCII characters to a variable.
# ----------------------------------------
quote=$'\042' # " assigned to a variable.
echo "$quote This is a 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 $ABC
echo; echo
escape=$'\033' # 033 is octal for escape.
echo "\"escape\" echoes as $escape"
# no visible output.
echo; echo
exit 0
See Example 34-1 for another example of the
$' ' string expansion
construct.
\"
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" # results in $variable01
\\
gives the backslash its literal meaning
echo "\\" # Results in \
# Whereas . . .
echo "\" # Invokes secondary prompt from the command line.
# In a script, gives an error message.
Elements of a string assigned to a variable may be escaped, but
the escape character alone may not be assigned to a variable.
variable=\
echo "$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, St�phane 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 St�phane Chazelas.
