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Full Discussion: Parsing null or empty output
Top Forums UNIX for Beginners Questions & Answers Parsing null or empty output Post 303009082 by Yoda on Friday 8th of December 2017 01:42:00 PM
Old 12-08-2017
In bash, for regular expression matching, you have to use a =~ regular expression matching operator

For checking if string is empty, you can use a -z string test operator

So you can try:-
Code:
if [[ "$lastseen" =~ '^[a-ZA-Z]*$' ]] && [ ! -z "$lastseen" ]
then
        echo "Serial Number Valid"
else
        echo "Serial Number Invaild"
        exit 3
fi

 

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STRING(3)						     Library Functions Manual							 STRING(3)

NAME
strcat, strncat, strcmp, strncmp, strcpy, strncpy, strlen, index, rindex - string operations SYNOPSIS
char *strcat(s1, s2) char *s1, *s2; char *strncat(s1, s2, n) char *s1, *s2; strcmp(s1, s2) char *s1, *s2; strncmp(s1, s2, n) char *s1, *s2; char *strcpy(s1, s2) char *s1, *s2; char *strncpy(s1, s2, n) char *s1, *s2; strlen(s) char *s; char *index(s, c) char *s, c; char *rindex(s, c) char *s; DESCRIPTION
These functions operate on null-terminated strings. They do not check for overflow of any receiving string. Strcat appends a copy of string s2 to the end of string s1. Strncat copies at most n characters. Both return a pointer to the null-termi- nated result. Strcmp compares its arguments and returns an integer greater than, equal to, or less than 0, according as s1 is lexicographically greater than, equal to, or less than s2. Strncmp makes the same comparison but looks at at most n characters. Strcpy copies string s2 to s1, stopping after the null character has been moved. Strncpy copies exactly n characters, truncating or null- padding s2; the target may not be null-terminated if the length of s2 is n or more. Both return s1. Strlen returns the number of non-null characters in s. Index (rindex) returns a pointer to the first (last) occurrence of character c in string s, or zero if c does not occur in the string. BUGS
Strcmp uses native character comparison, which is signed on PDP11's, unsigned on other machines. STRING(3)
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