You’ve asked for input (e.g., from a user or a program) and to ensure security or data integrity you need to make sure you got what you asked for.

There are various ways to validate your input, depending on what the input is and how strict you need to be.

Use pattern matching for simple “it matches or it doesn’t” situations (see Testing for Equal, Testing with Pattern Matches, and Testing with Regular Expressions).

[[ "$raw_input" == *.jpg ]] && echo "Got a JPEG file."

Use a case statement when there are various things that might be valid (see Branching Many Ways and Parsing Command-Line Arguments).

# cookbook filename: validate_using_case

case $raw_input in
    *.company.com        ) # Probably a local hostname
        ;;
    *.jpg                ) # Probably a JPEG file
        ;;
    *.[jJ][pP][gG]       ) # Probably a JPEG file, case insensitive
        ;;
    foo | bar            ) # entered 'foo' or 'bar
        ;;
    [0-9][0-9][0-9]      ) # A 3 digit number
        ;;
    [a-z][a-z][a-z][a-z] ) # A 4 lower-case char word
        ;;
    *                    ) # None of the above
        ;;
esac

Use a regular expression when pattern matching isn’t specific enough and you have bash version 3.0+ (see Testing with Regular Expressions). This example is looking for a three to six alphanumeric character filename with a .jpg extension (case sensitive):

[[ "$raw_input" =~ [[:alpha:]]{3,6}.jpg ]] && echo "Got a JPEG file."

For a larger and more detailed example, see the examples/scripts/shprompt in a recent bash tarball. Note this was written by Chet Ramey, who maintains bash:

# shprompt -- give a prompt and get an answer satisfying certain criteria
#
# shprompt [-dDfFsy] prompt
#    s = prompt for string
#    f = prompt for filename
#    F = prompt for full pathname to a file or directory
#    d = prompt for a directory name
#    D = prompt for a full pathname to a directory
#    y = prompt for y or n answer
#
# Chet Ramey
# [email protected]

For a similar example, see examples/scripts.noah/y_or_n_p.bash written circa 1993 by Noah Friedman and later converted to bash version 2 syntax by Chet Ramey. Also in the examples see: ./functions/isnum.bash, ./functions/isnum2, and ./functions/isvalidip.