How I can use 'grep' command to grab some value from the arguments?

Question

Actually, I am doing some assignment by follow:

1: Write a program called valid that prints "yes" if its argument is a valid shell variable name and "no" otherwise:

What I am doing is I want to find some value which includes some regular expression, such as [0-9]* by using 'grep' command. But I have no idea how to grab some value including that expression from the argument I input, since 'grep' command is basically to capture some line in the file. Any help will be really appreciated

Answer 1

This doesn't use grep, but as a point of reference, you could use bash's =~ conditional operator to compare the script's first argument with the regular expression class for a name, which is defined by the Bash Reference Manual as:

A word consisting solely of letters, numbers, and underscores, and beginning with a letter or underscore. Names are used as shell variable and function names.

$ cat isvarname
#!/bin/bash
if [ "$#" -ne 1 ]
then
   echo "Usage: $0 a-string"
   exit 1
fi

if [[ "$1" =~ ^[[:alpha:]_][[:alnum:]_]*$ ]]
then
  echo yes
else
  echo no
fi

Answer 2

In bash a valid variable name is made of one or more single-byte characters with the first one being alphabetical or underscore, and the remaining ones if any being alphabetical, 0123456789 or underscore.

For instance Stéphane is a valid variable name only in locales where é is single-byte like in ISO-8859-1 where it's the 0xE9 byte, not in UTF-8 where it's encoded as 0xC3 0xA9.

You could do something like:

#! /usr/bin/env bash
is_single_byte() {
  local length_in_bytes length_in_chars
  length_in_chars=${#1}
  local LC_ALL=C
  length_in_bytes=${#1}
  ((length_in_bytes == length_in_chars))
}

re='^[[:alpha:]_][[:alnum:]_]*$'
for var do
  if is_single_byte "$var" && [[ $var =~ $re ]]; then
    printf '"%s" is a valid variable name\n' "$var"
  else
    printf '"%s" is not a valid variable name\n' "$var"
  fi
done

Answer 3

#!/bin/bash
var=$1
if [[ $var =~ [\`\$\=\/] || $var =~ \[ ]]; then
    echo 'no'
    exit
fi
declare "$var"=test 2>/dev/null
if [[ ${!var} == 'test' || $1 == 'var' ]]; then
    echo 'yes'
else
    echo 'no'
fi

This will try to assign the value test to the provided value. If it succeeds (if the value is a valid variable name) it will match the if test and echo yes, else it will echo no.

Answer 4

The first command line argument is available as $1. A valid shell variable name starts with a alphabetic character (or underscore) and continues with alphanumerics (or underscores).

Two shell patterns that matches an invalid shell variable name is

[!a-zA-Z_]*

and

[a-zA-Z_]*[!a-zA-Z_0-9]*

You may use case ... esac to do pattern matching against a variable's value.

Spoiler warning:

#!/bin/sh
LC_ALL=C
case "$1" in
    [!a-zA-Z_]*|[a-zA-Z_]*[!a-zA-Z_0-9]*|"")
        echo 'NO'
        ;;
    *)
        echo 'YES'
esac

This also answers "NO" for a variable whose name is empty. Note that this is using shell globbing patterns, not regular expressions, and that it runs in any POSIX shell, not just bash.

Testing:

$ ./script.sh _ae
YES
$ ./script.sh 0a9oe
NO
$ ./script.sh aoeat
YES
$ ./script.sh aoeat-aoe
NO