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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML
><HEAD
><TITLE
>Escaping</TITLE
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>Advanced Bash-Scripting Guide: </TH
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>Chapter 5. Quoting</TD
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="ESCAPINGSECTION"
></A
>5.2. Escaping</H1
><P
><A
NAME="ESCP"
></A
><I
CLASS="FIRSTTERM"
>Escaping</I
> is a method
of quoting single characters. The <SPAN
CLASS="TOKEN"
>escape</SPAN
>
(<SPAN
CLASS="TOKEN"
>\</SPAN
>) preceding a character tells the shell to
interpret that character literally.</P
><DIV
CLASS="CAUTION"
><P
></P
><TABLE
CLASS="CAUTION"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="../images/caution.gif"
HSPACE="5"
ALT="Caution"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>With certain commands and utilities, such as <A
HREF="internal.html#ECHOREF"
>echo</A
> and <A
HREF="sedawk.html#SEDREF"
>sed</A
>, escaping a character may have the
opposite effect - it can toggle on a special meaning for that
character.</P
></TD
></TR
></TABLE
></DIV
><P
></P
><DIV
CLASS="VARIABLELIST"
><P
><B
><A
NAME="SPM"
></A
>Special meanings of certain
escaped characters</B
></P
><DL
><DT
>used with <B
CLASS="COMMAND"
>echo</B
> and
<B
CLASS="COMMAND"
>sed</B
></DT
><DD
><P
></P
></DD
><DT
><SPAN
CLASS="TOKEN"
>\n</SPAN
></DT
><DD
><P
>means newline</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>\r</SPAN
></DT
><DD
><P
>means return</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>\t</SPAN
></DT
><DD
><P
>means tab</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>\v</SPAN
></DT
><DD
><P
> means vertical tab</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>\b</SPAN
></DT
><DD
><P
>means backspace</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>\a</SPAN
></DT
><DD
><P
>means <I
CLASS="FIRSTTERM"
>alert</I
> (beep or flash)</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>\0xx</SPAN
></DT
><DD
><P
><A
NAME="OCTALREF"
></A
>translates to the
octal <A
HREF="special-chars.html#ASCIIDEF"
>ASCII</A
>
equivalent of <TT
CLASS="REPLACEABLE"
><I
>0nn</I
></TT
>, where
<TT
CLASS="REPLACEABLE"
><I
>nn</I
></TT
> is a string of digits</P
><DIV
CLASS="IMPORTANT"
><P
></P
><TABLE
CLASS="IMPORTANT"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="../images/important.gif"
HSPACE="5"
ALT="Important"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
><A
NAME="STRQ"
></A
></P
><P
>The <TT
CLASS="USERINPUT"
><B
>$' ... '</B
></TT
>
<A
HREF="quoting.html#QUOTINGREF"
>quoted</A
> string-expansion
construct is a mechanism that uses escaped octal or hex values
to assign ASCII characters to variables, e.g.,
<B
CLASS="COMMAND"
>quote=$'\042'</B
>.</P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="EXAMPLE"
><A
NAME="ESCAPED"
></A
><P
><B
>Example 5-2. Escaped Characters</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>#!/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.
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 $escape"
# no visible output.
echo
exit 0</PRE
></FONT
></TD
></TR
></TABLE
></DIV
><P
>A more elaborate example:</P
><DIV
CLASS="EXAMPLE"
><A
NAME="BASHEK"
></A
><P
><B
>Example 5-3. Detecting key-presses</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>#!/bin/bash
# Author: Sigurd Solaas, 20 Apr 2011
# Used in ABS Guide with permission.
# Requires version 4.2+ of Bash.
key="no value yet"
while true; do
clear
echo "Bash Extra Keys Demo. Keys to try:"
echo
echo "* Insert, Delete, Home, End, Page_Up and Page_Down"
echo "* The four arrow keys"
echo "* Tab, enter, escape, and space key"
echo "* The letter and number keys, etc."
echo
echo " d = show date/time"
echo " q = quit"
echo "================================"
echo
# Convert the separate home-key to home-key_num_7:
if [ "$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
;;
$'\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 $?</PRE
></FONT
></TD
></TR
></TABLE
></DIV
><P
>See also <A
HREF="bashver2.html#EX77"
>Example 37-1</A
>.</P
></DD
><DT
><SPAN
CLASS="TOKEN"
>\"</SPAN
></DT
><DD
><P
> gives the quote its literal meaning</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>echo "Hello" # Hello
echo "\"Hello\" ... he said." # "Hello" ... he said.</PRE
></FONT
></TD
></TR
></TABLE
></P
></DD
><DT
><SPAN
CLASS="TOKEN"
>\$</SPAN
></DT
><DD
><P
>gives the dollar sign its literal meaning
(variable name following <SPAN
CLASS="TOKEN"
>\$</SPAN
> will not be
referenced)</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>echo "\$variable01" # $variable01
echo "The book cost \$7.98." # The book cost $7.98.</PRE
></FONT
></TD
></TR
></TABLE
></P
></DD
><DT
><SPAN
CLASS="TOKEN"
>\\</SPAN
></DT
><DD
><P
>gives the backslash its literal meaning</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>echo "\\" # Results in \
# Whereas . . .
echo "\" # Invokes secondary prompt from the command-line.
# In a script, gives an error message.
# However . . .
echo '\' # Results in \</PRE
></FONT
></TD
></TR
></TABLE
></P
></DD
></DL
></DIV
><DIV
CLASS="NOTE"
><P
></P
><TABLE
CLASS="NOTE"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="../images/note.gif"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The behavior of <SPAN
CLASS="TOKEN"
>\</SPAN
> depends on whether
it is escaped, <A
HREF="varsubn.html#SNGLQUO"
>strong-quoted</A
>,
<A
HREF="varsubn.html#DBLQUO"
>weak-quoted</A
>, or appearing within
<A
HREF="commandsub.html#COMMANDSUBREF"
>command substitution</A
> or a
<A
HREF="here-docs.html#HEREDOCREF"
>here document</A
>.
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
> # Simple escaping and quoting
echo \z # z
echo \\z # \z
echo '\z' # \z
echo '\\z' # \\z
echo "\z" # \z
echo "\\z" # \z
# Command substitution
echo `echo \z` # z
echo `echo \\z` # z
echo `echo \\\z` # \z
echo `echo \\\\z` # \z
echo `echo \\\\\\z` # \z
echo `echo \\\\\\\z` # \\z
echo `echo "\z"` # \z
echo `echo "\\z"` # \z
# Here document
cat &#60;&#60;EOF
\z
EOF # \z
cat &#60;&#60;EOF
\\z
EOF # \z
# These examples supplied by St<53>phane Chazelas.</PRE
></FONT
></TD
></TR
></TABLE
>
</P
><P
>Elements of a string assigned to a variable may be escaped, but
the escape character alone may not be assigned to a variable.
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>variable=\
echo "$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.</PRE
></FONT
></TD
></TR
></TABLE
>
</P
></TD
></TR
></TABLE
></DIV
><P
>Escaping a space can prevent word splitting in a command's argument list.
<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>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.</PRE
></FONT
></TD
></TR
></TABLE
></P
><P
><A
NAME="ESCNEWLINE"
></A
></P
><P
>The <SPAN
CLASS="TOKEN"
>escape</SPAN
> also provides a means of writing a
multi-line command. Normally, each separate line constitutes
a different command, but an <SPAN
CLASS="TOKEN"
>escape</SPAN
> at the end
of a line <EM
>escapes the newline character</EM
>,
and the command sequence continues on to the next line.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>(cd /source/directory &#38;&#38; tar cf - . ) | \
(cd /dest/directory &#38;&#38; 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<53>phane Chazelas.)</PRE
></FONT
></TD
></TR
></TABLE
>
<DIV
CLASS="NOTE"
><P
></P
><TABLE
CLASS="NOTE"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="../images/note.gif"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>If a script line ends with a <SPAN
CLASS="TOKEN"
>|</SPAN
>, a pipe
character, then a <SPAN
CLASS="TOKEN"
>\</SPAN
>, 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.</P
></TD
></TR
></TABLE
></DIV
></P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><FONT
COLOR="#000000"
><PRE
CLASS="PROGRAMLISTING"
>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<53>phane Chazelas.</PRE
></FONT
></TD
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></TABLE
></P
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