Difference between revisions of "Quoting"

From Linux Shell Scripting Tutorial - A Beginner's handbook
Jump to navigation Jump to search
Line 90: Line 90:
         [ "$SERVER_MODE"  = "clean" \
         [ "$SERVER_MODE"  = "clean" \
           -a -e /usr/local/etc/nixcraft/lighttpd/disk_cache.init ] && return 0
           -a -e /usr/local/etc/nixcraft/lighttpd/disk_cache.init ] && return 0
         return 1
         return 0

Revision as of 14:50, 29 September 2011

← Echo CommandHomeExport Variables →

Your bash shell understands special characters with special meanings. For example, $var is used to expand the variable value. Bash expands variables and wildcards, for example:

echo "$PATH"
echo "$PS1"
echo /etc/*.conf

However, sometime you do not wish to use variables or wildcards. For example, do not print value of $PATH, but just print $PATH on screen as a word. You can enable or disable the meaning of a special character by enclosing them in single quotes. This is also useful to suppress warnings and error messages while writing the shell scripts.

echo "Path is $PATH"  ## $PATH will be expanded


echo 'I want to print $PATH' ## PATH will not be expanded


There are three types of quotes:

Quote type Name Meaning Example (type at shell prompt)
" The double quote The double quote ( "quote" ) protects everything enclosed between two double quote marks except $, ', " and \.Use the double quotes when you want only variables and command substitution.
* Variable - Yes
* Wildcards - No
* Command substitution - yes
The double quotes allowes to print the value of $SHELL variable, disables the meaning of wildcards, and finally allows command substitution.
echo "$SHELL"
echo "/etc/*.conf"
echo "Today is $(date)"

' The single quote The single quote ( 'quote' ) protects everything enclosed between two single quote marks. It is used to turn off the special meaning of all characters.
* Variable - No
* Wildcards - No
* Command substitution - No
The single quotes prevents displaying variable $SHELL value, disabled the meaning of wildcards /etc/*.conf, and finally command substitution ($date) itself.
echo '$SHELL'
echo '/etc/*.conf'
echo 'Today is $(date)'

\ The Backslash Use backslash to change the special meaning of the characters or to escape special characters within the text such as quotation marks. You can use \ before dollar sign to tell the shell to have no special meaning. Disable the meaning of the next character in $PATH (i.e. do not display value of $PATH variable):
echo "Path is \$PATH"
echo "Path is $PATH"

The Backslash

The backslash ( \ ) alters the special meaning of the ' and " i.e. it will escape or cancel the special meaning of the next character. The following will display filename in double quote:

echo "File is \"$FILE\" "

Sample Outputs:

File is "/etc/resolv.conf"

The following will remove the special meaning of the dollar ( $ ) sign:

echo "File is \$FILE "

Sample Outputs:

File is $FILE 

The backslash-escaped characters

You can use the following backslash-escaped characters. It will get replaced as specified by the ANSI C standard. Quoting directly from bash man page:

              \a     alert (bell)
              \b     backspace
              \e     an escape character
              \f     form feed
              \n     new line
              \r     carriage return
              \t     horizontal tab
              \v     vertical tab
              \\     backslash
              \'     single quote
              \nnn   the eight-bit character whose value is the octal value nnn (one to three digits)
              \xHH   the eight-bit character whose value is the hexadecimal value HH (one or two hex digits)
              \cx    a control-x character

Use -e option of echo command to enable interpretation of backslash escapes.


echo "Pizza bill \$22.5"
echo -e "\a Ding dong\a"
echo "CIFS path must be \\\\NT-Server-Name\\ShareName"
echo -e "Sr.no\t DVD (price) "
echo -e "1\t Spirited Away (INR.200)"
echo -e "2\t Dragon Ball Z (INR.300)"

The special parameters * and @ have special meaning when in double quotes, but you can disable them with the backslash:

echo "*"
echo "\*"
echo "\@"

Continue command on next line

You can use the backslash (\) as last character on line to continue command on next line:

echo "A monkey-tailed boy named Goku is found by an old martial \
>arts expert who raises him as his grandson. One day Goku meets a \
>girl named Bulma and together they go on a quest to retrieve the seven Dragon Balls"

You can also use the backslash while writing program or function:

# Purpose: clean /tmp/$domain ?
check_temp_clean() {
        [ "$SERVER_MODE" = "daemon" ] || return 1
        [ "$SERVER_MODE"  = "init"    ] && return 0
        # note use of the backslash character to continue command on next line    
        [ "$SERVER_MODE"  = "clean" \
          -a -e /usr/local/etc/nixcraft/lighttpd/disk_cache.init ] && return 0
        return 0

Protecting command line arguments

Type the following command to find out all c program file (*.c):

find $HOME -name *.c

In the above example, the *.c is expanded by the bash shell. It will try to match all filename ending with .c in the /home directory (current user's home directory) such as main.c, lib1.c, lib2.c, ssh.c, auth.c etc. You can escape the wild card using the backslash as the escape character:

find $HOME -name \*.c
find $HOME -name \*main.c
find /nas01/apps/admin -iname \*py

You can also use the double quote

find $HOME -name "*.c"

← Echo CommandHomeExport Variables →