How do I make an output for grep fail?

Question

I am using if-else statement to search for keywords and displaying the results in the terminal, here's an example of my code.

read finding

if ["$finding" != "" ]; then
   grep $finding information.txt
else
   echo "No such information in database."
fi

But the terminal does not display anything if i key in information that does not exist. I started shell about a week back, might need more explanation on how certain code works.

Answer 1

• Add space after [ (it is a command)
• Use -n to test if length of string is nonzero, or -z to test if it is zero
• Put double quotes around variables

So:

read finding

if [ -z "$finding" ]; then
    echo "You didn't enter anything"
else
    grep "$finding" information.txt
    if [ ! "$?" -eq 0 ]; then
        echo "No such information in database."
    fi
fi

Answer 2

Using some logic operators (Starting from @jimmij)

grep -q "$finding" information.txt behaves like a boolean answer (prints nothing just has a return status of 0 if found, non-zero otherwise).

grep -F -- "$finding" ... search text instead of regexp (remove it if you want to search for regexp -- there may be some security risks... (thank you @StephanChazelas))

read finding

if [ -z "$finding" ]; then
   echo "You didn't enter anything"
else
   grep -qF -- "$finding" information.txt   && echo "found"   || echo "not found"
fi

or even

[ -z "$finding" ] && 
     echo "nothing entered" || 
     grep -qF -- "$finding" information.txt || echo "not found"

(though beware it will also run the second grep and/or echo if the first echo fails)

Edit 1 > explain grep -q ... Sure.

In normal situations, grep return status is 0 (and just returns "not 0" if an error occurs (eg. file not found))

grep -qF exp file "returns" 0 if it finds exp in file, error otherwise (grep -q exp file would do that if the exp regexp was matched in file).

This behavior can be used in bash control statements (if,elif, while, &&, ||,etc)

read f

if [ -z "$f" ]; then
   echo "You didn't enter anything"
elif grep -qF -- "$f" information.txt; then
   echo "found"
else
   echo "not found"
fi

Answer 3

i=0
until  [ "$((i+=1))" -gt 5 ] || {
       printf '\nPrompt: '
       read finding && [ -n "$finding" ]
};do   printf '\nINVALID:\tTry again.\n'
done   <>/dev/tty >&0
grep -- "${finding:?INSUFFICIENT INPUT!}" <info.txt ||
! printf '%s:\tNot found in database.' "$finding" >&2

You should be testing not only that $finding is not null, but also that the read is completing successfully - and so setting it in the first place. The above loop will prompt on and attempt to read from the terminal a line of not-null input input 5 times before quitting w/ error.

• "${expand:?SUBSTITUTE ONLY IF UNSET OR NULL! >STDERR}"
  • An empty value for $finding is handled as a side effect of the ${parameter:?expansion} and results in a message written to stderr and the immediate exit of a non-interactive shell.
• cmd ... <redirected_input_file
  • It is a syntax error to attempt to redirect from a file which cannot be opened (such as one which does not exist or for which the effective UID/GID does not have read permissions) and so if info.txt is for whatever reason invalid a non-interactive shell writes a diagnostic message to stderr and also exits immediately. This also holds true if the attempted read exceeds any limitations as effected by ulimit - and applies equally for the initial until ... done <>in >&out loop.
• grep ... || ! printf '%s: fmt' "$param"
  • Because any user or file input errors have already resulted in a shell exit before grep is called, the only two other possible results of the above form are that grep matches the pattern in "$finding" at least once and writes the corresponding lines to stdout or that it does not match even once and returns other than 0. In the latter case printf writes an informative message to stderr and returns 1.