mjhjmtu

condition
string either start with upper or lower alphabets or underscore
for example _abc or xyz or Abc

string may contain digits after first character
like _f9 or f10 or car20 or top10cars

string should never contain special characters like ! @ # $ % ^ & * ( ) + - =

here my tiny script

if [[ $1 =~ ^[A-Za-z_]+$ ]]; then
 echo it matches
else
 echo does_not match
fi

when i pass argument like only & or * or ( , script don't work , sometimes cursor never comes backs , i need to press ctrl+c

for example ./script.sh (abc 
 ./script.sh &&
 ./script.sh &abc

whats wrong with script

Quoting.

In the script, use "$1" rather than just $1.

On the command line, use

./script '*(ontehu'

instead of

./script *(ontehu

• ./script.sh (abc This is a syntax error in the shell grammar.




• ./script.sh && This makes the shell think there is a conditional AND on the command line and it expects something on the right hand side of the && operator.




• ./script.sh &abc This is two commands: ./script started as a background process (with &), and the command abc.

In all these cases, the argument should be quoted (single quoted, ideally, unless you need the shell to insert the value of a shell variable, in which case it should be double quoted).

Your script:

if [[ $1 =~ ^[A-Za-z_]+$ ]]; then
 echo it matches
else
 echo does_not match
fi

Quote $1:

if [[ "$1" =~ ^[A-Za-z_]+$ ]]; then
 echo it matches
else
 echo does_not match
fi

Allow digits in the tail end of the value:

if [[ "$1" =~ ^[A-Za-z_][A-Za-z0-9_]+$ ]]; then
 echo it matches
else
 echo does_not match
fi

Do proper reporting of errors (this is extra):

if [[ "$1" =~ ^[A-Za-z_][A-Za-z0-9_]+$ ]]; then
 printf '"%s" is a valid variable namen' "$1"
else
 printf '"%s" is not a proper variable namen' "$1" >&2
 exit 1
fi

That's because these are reserved characters.

& means run command in the background

* resolves to all files/dirs in the actual directory, which are then passed as arguments

() is used for command order preference or function declarations

If you want such a characters in the string, put it in quotes " " or ' '