Shell tutorial 4-shell replacement

If the expression contains special characters, shell will replace it. For example, using variables in double quotes is a replacement, and escape characters are also a replacement.

For example:

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1. #! /Bin/bash
2.  
3. A = 10
4. Echo-e "value of A is $ A \ n"

#!/bin/basha=10echo -e "Value of a is $a \n"

Running result:

Value of a is 10

Here,-E replaces the escape characters. If the-e option is not used, the output is as follows:

Value of a is 10\n

The following escape characters can be used in ECHO:

Escape characters	Description
\\	Backslash
\	Alarm, bell
\ B	Return (delete key)
\ F	Change page (ff), move the current position to the beginning of the next page
\ N	Line feed
\ R	Enter
\ T	Horizontal tab (Tab key)
\ V	Vertical Tab

You can use the-E Option of the ECHO command to disable escape. By default, escape is not allowed. You can use the-n option to disable line breaks.

Command replacement

Command replacement means that the shell can first execute the command to temporarily Save the output result and output it in a proper place.

Syntax of command replacement:

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1. 'Command'

`command`

Note that the quotation marks are not single quotes. The key is located below the ESC key.

In the following example, the command execution result is saved in the variable:

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1. #! /Bin/bash
2.  
3. Date = 'date'
4. Echo "date is $ date"
5.  
6. Users = 'who | WC-l'
7. Echo "logged in user are $ users"
8.  
9. Up = 'date; uptime'
10. Echo "uptime is $ up"

#!/bin/bashDATE=`date`echo "Date is $DATE"USERS=`who | wc -l`echo "Logged in user are $USERS"UP=`date ; uptime`echo "Uptime is $UP"

Running result:

Date is Thu Jul  2 03:59:57 MST 2009Logged in user are 1Uptime is Thu Jul  2 03:59:57 MST 200903:59:57 up 20 days, 14:03,  1 user,  load avg: 0.13, 0.07, 0.15

Variable replacement

Variable replacement can be used to change the value of a variable based on its State (whether it is empty or defined ).

Possible variable replacement formats:

Form	Description
$ {Var}	Original variable value
$ {Var:-word}	If the VaR variable is null or has been deleted (unset), the return word will not change the value of var.
$ {Var: = word}	If the VaR variable is null or has been deleted (unset), the system returns the word and sets the VaR value to word.
$ {Var :? Message}	If the variable VAR is null or has been deleted (unset), send the message to the standard error output to check whether the variable VAR can be assigned a value normally. If this replacement occurs in the shell script, the script stops running.
$ {Var: + word}	If the variable VAR is defined, word is returned, but the value of VaR is not changed.

See the following example:

#!/bin/bashecho ${var:-"Variable is not set"}echo "1 - Value of var is ${var}"echo ${var:="Variable is not set"}echo "2 - Value of var is ${var}"unset varecho ${var:+"This is default value"}echo "3 - Value of var is $var"var="Prefix"echo ${var:+"This is default value"}echo "4 - Value of var is $var"echo ${var:?"Print this message"}echo "5 - Value of var is ${var}"

Running result:

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1. Variable is not set
2. 1-value of VaR is
3. Variable is not set
4. 2-value of VaR is variable is not set
5.  
6. 3-value of VaR is
7. This is default value
8. 4-value of VaR is prefix
9. Prefix
10. 5-value of VaR is prefix

Shell tutorial 4-shell replacement