Difference between revisions of "Quoting"

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| The single quotes prevents displaying variable $SHELL value, disabled the meaning of [[wildcards]] /etc/*.conf, and finally command substitution ($date) itself. <br/><code>echo '$SHELL'<br/>echo '/etc/*.conf'<br />echo 'Today is $(date)'</code><br />&nbsp;
 
| The single quotes prevents displaying variable $SHELL value, disabled the meaning of [[wildcards]] /etc/*.conf, and finally command substitution ($date) itself. <br/><code>echo '$SHELL'<br/>echo '/etc/*.conf'<br />echo 'Today is $(date)'</code><br />&nbsp;
 
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|`
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|Back quote
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|Backslahs
|Use back quote ( `command-name` ) to '''execute command''' and replace a command with its output within the same command-line. However, ''$(command-name)'' is encouraged syntax for substitution as it is recommended by [[POSIX]] standard and it improves script readability.<br/>* '''Variable''' - Yes<br/>* '''Wildcards''' - Yes<br/>* '''Command substitution''' - yes
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|Use backslahs to change the special meaning of the characters or to escape special characters within the text such as quotation marks.
|The [[date command]] is executed and its output is substituted back to [[echo command]].<br/><code>echo "Today is `date`"<br />echo "Today is $(date)"<br/>echo "$(ls /etc/*.conf)"<br/>FILES=/etc/resolv.conf<br/>echo "$(cat $FILE)"</code><br />&nbsp;
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|You can use \ before dollar sign is used to told to have no special meaning. Disable the meaning of the next character in $PATH (i.e. do not display value of $PATH variable):<br/><code>echo "Path is \$PATH"<br />echo "Path is $PATH"<br />&nbsp;
 
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Revision as of 16:28, 19 October 2009

← Echo CommandHomeExport Variables →

Your bash shell understand special characters with special meanings. For example, $var is used to display 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 into a single or double quotes. This is also useful to suppress warnings and error messages while writing the shell scripts.

echo "Path is $PATH"

OR

echo 'I want to print $PATH'

Quoting

There are three types of quotes

Quote type Name Meaning Example (type at shell prompt)
" Double Quotes 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)"

 
' Single quotes 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)'

 
\ Backslahs Use backslahs 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 is used to told 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"
 

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:

FILE="/etc/resolv.conf"
echo "File is \"$FILE\" "

Sample Outputs:

File is "/etc/resolv.conf"

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

FILE="/etc/resolv.conf"
echo "File is \$FILE "

Sample Outputs:

File is $FILE 

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