Shell Advanced Variable operation

first, built-in variables 1. $BASH

The path to the bash's binary program file (the path to the script interpreter)

$ vim test.sh

Enter the code:

#!/bin/bash
Echo $BASH

To run the code:

$ bash test.sh

2. $FUNCNAME

The name of the current function

$ vim test.sh

Enter the code:

#!/bin/bash
xyz23 ()
{
  echo "$FUNCNAME now executing."
  # Normal Print: xyz23 now executing.
  # in individual versions of Bash, $funcname built-in variables are not supported
}

xyz23

echo "FUNCNAME = $FUNCNAME"        # FUNCNAME =
# Exceeding the scope of the function becomes a null value.

To run the code:

$ bash test.sh

3. $IFS

The internal domain delimiter, which is used to determine how Bash recognizes a field or word boundary when interpreting a string.

$IFS defaults to whitespace (spaces, tabs, and line breaks), but this can be modified, for example, when parsing a comma

Data file, you can set it to a comma, noting that $* is using the first character saved in $ifs.

$ vim test.sh

Enter the code:

#!/bin/bash
# $IFS processing whitespace differs from processing other characters.

Output_args_one_per_line ()
{for
  arg does
  echo "[$arg]"
  done
}

echo; echo "ifs=\" \ ""
Echo "-------"

ifs= ""
var= "a  b C   "
output_args_one_per_line $var  # Output_args_one_per_ Line ' echo ' a  b C   "'
# #
[A]
# [b]
# [C]


echo; echo" ifs=: "
echo"-----"

ifs=:
var= ": a::b:c:::"               # as above, but with "" replaced with ":".
Output_args_one_per_line $var # # []
# [
a] # [] # [
b] # [
c] # []
# []
# []

# The same thing can happen in Awk's "FS" domain.

# Thanks, Stephane Chazelas.

Echo

exit 0

Note: You may have questions about the For loop and the variable arg above, which will be discussed in the next section.

To run the code:

$ bash test.sh

4. $REPLY

When no parameter variable is supplied to the Read command, the variable is supplied to the Read command as a default variable. Also

Can be used with the Select menu, but only provide the number of the selected variable, not the value of the variable itself. Built-in variable $reply is not supported in individual versions of Bash.

$ vim test.sh

Enter the code:

#!/bin/bash
# reply.sh

# reply is the default variable provided to the ' read ' command.

echo
echo-n "What's your favorite vegetable?"
Read

echo "Your favorite vegetable is $REPLY."
#
#+ reply saves the value read by the last "read" command when and only if no variable is provided to the "read" command.

echo
echo-n "What's your favorite fruit?"
Read Fruit
echo "Your favorite fruit is $fruit."
echo "But ..."
echo "Value of \ $REPLY is still $REPLY."
#  $REPLY still holds the value of the previous Read command,
#+ because the variable $fruit is passed into the new "read" command.

Echo

exit 0

To run the code:

$ bash test.sh

5. List all the parameters by $* and $@

All positional parameters of "$*" are considered as a single word.

$@ is the same as $*, but each parameter is a separate reference string, which means that the parameter is fully

Passed, and was not interpreted or extended. This also means that each parameter in the argument list is treated as a separate word.

$ vim test.sh

Enter the code:

#!/bin/bash # uses several parameters to invoke this script, such as "one and three". e_badargs=65 if [!-N "$] then echo" Usage: ' basename ' argument1 argument2 etc "Exit $E _badargs fi echo Index

=1 # Start Count.
echo "Listing args with \" \$*\ ":" For Arg in "$*" # If "$*" is not referenced by "" "(double quotes), then it will not work properly.
Do Echo ' Arg # $index = $arg ' let ' index+=1 "done # $* all arguments as one word.

echo "entire arg list seen as single word."

echo index=1 # Reset Count (Translator Note: starting from 1). Echo ' Listing args with \ ' \$@\ ': ' for arg in ' $@ ' do Echo ' arg # $index = $arg ' let ' index+=1 ' done # $@ put each parameter
As a single word.

echo "Arg list seen as separate words."

echo index=1 # Reset Count (Translator Note: starting from 1). echo "Listing args with \$* (unquoted): ' For ARG in $* do Echo ' arg # $index = $arg ' let ' index+=1 ' done # unreferenced
$* will consider the parameters as separate words.

echo "Arg list seen as separate words." Exit 0

To run the code:

$ bash test.sh

second, the operation string 1. String length

$ vim test.sh

Enter the code:

#!/bin/bash

stringz=abcabc123abcabc

echo ${#stringZ}                 #
expr length $stringZ             # 15

To run the code:

$ bash test.sh

2. Using awk to process strings

The bash script can also invoke awk's string manipulation function in place of its own built-in string operation.

Example: Extracting a String

$ vim test.sh

Enter the code:

#!/bin/bash

string=23skidoo1
#      012345678    Bash
#      123456789    awk
# Note the different string indexing system:
# The first character of Bash is recorded from ' 0 '.
# The first character of Awk is recorded starting with ' 1 '.

Echo ${string:2:4} # Position 3 (0-1-2), 4 character
# skid

# The command equivalent to ${string:pos:length in awk is substr (string,pos,length). C13/>echo | awk '
{
    print substr ("'" ${string} "'", 3,4)      # skid
}
'
#  use an empty "echo" Pass the pipeline to awk with a false input,
#+ so you don't have to provide a file name to awk.

Exit 0

To run the code:

$ bash test.sh