Escaping
| Bash Unix Shell Scripting: | ||
|---|---|---|
| Quoting Variables | Chapter 5. Quoting | Exit and Exit Status |
Escaping
Escaping is a method of quoting single characters. The escape (\) preceding a character tells the shell to interpret that character literally.
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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
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means newline
- \r
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means return
- \t
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means tab
- \v
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means vertical tab
- \b
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means backspace
- \a
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means alert (beep or flash)
- \0xx
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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. 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 $escape" # no visible output. echo exit 0Example 5-3. Detecting key-presses
#!/bin/bash # Author: Sigurd Solaas, 20 Apr 2011 # Used in ABS Guide with permission. # Requires version 4.2+ of Bash. key="no value yet" while true; do clear echo "Bash Extra Keys Demo. Keys to try:" echo echo "* Insert, Delete, Home, End, Page_Up and Page_Down" echo "* The four arrow keys" echo "* Tab, enter, escape, and space key" echo "* The letter and number keys, etc." echo echo " d = show date/time" echo " q = quit" echo "================================" echo # Convert the separate home-key to home-key_num_7: if [ "$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 ;; $'\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.
- \"
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gives the quote its literal meaning
echo "Hello" # Hello echo "\"Hello\" ... he said." # "Hello" ... he said.
- \$
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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.
- \\
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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 \
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The behavior of \ depends on whether it is escaped, strong-quoted, weak-quoted, or appearing within command substitution or a here document.
Elements of a string assigned to a variable may be escaped, but the escape character alone may not be assigned to a variable.
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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.) |
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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 Stéphane Chazelas. |