How can I obtain command returning status in linux shell

Question

Say, I call grep "blabla" $file in my shell. How could I know whether grep found "blabla" in $file?

I try ret='grep "blabla" $file' , will this work by viewing the value of ret ? If yes, is ret integer value or string value or something else?

Answer 1

If you do exactly

ret='grep "blabla" $file'

then ret will contain the string "grep "blabla" $file".

If you do (what you meant)

ret=`grep "blabla" $file`

then ret will contain whatever output grep spit out (the lines that matched "blabla" in $file ).

If you just want to know whether grep found any lines that matched "blabla" in $file then you want to check the return code of grep -q blabla "$file" (note that you don't need to quote literal strings when they don't contain special characters and that you should quote variable references).

The variable $? contains the return code of the most recently executed command. So

grep -q blabla "$file"
echo "grep returned: $?"

will echo the return code from grep which will be 0 if any lines were output.

The simplest way to test that and do something about it is, however, not to use $? at all but instead to just embed the grep call in an if statement like this

if grep -q blabla "$file"; then
    echo grep found something
else
    echo grep found nothing
fi

Answer 2

When you run the command

grep blabla "$file"

Status is saved in the variable $? . 0 is good, greater than 0 is bad. So you could do

grep -q blabla "$file"
if [ $? = 0 ]
then
  echo found
fi

Or save to a variable

grep -q blabla "$file"
ret=$?
if [ $ret = 0 ]
then
  echo found
fi

Or just use if with grep directly

if grep -q blabla "$file"
then
  echo found
fi