Redirecting Grep Error Output to a File

grepoutputstderr

I wanted to capture to a file the errors being returned on the command line from grep. For example,

grep foo.lookup
No such file in directory

I want to output that to a log file. This is my shell script:

lookUpVal=1
var1=$(grep $lookUpVal foo.lookup) >>lookup.log 2>$1

It creates the file lookup.log but doesn't write the error on it.

Best Answer

If I understand it correct, you want to capture the output of grep into a variable and append any error to the logfile.

You could say:

var1=$(grep $lookUpVal foo.lookup 2>>lookup.log)

The $(...) syntax denotes command substitution, i.e. outputs the result of the command into a variable. By default it would capture the STDOUT of the command into the variable and the STDERR is printed to the console. In order to redirect the STDERR to a file, you would need to perform the redirection within the command itself, i.e. within $(...).

Related Solutions

Shell – grep: Not a recognized flag in AIX

Looks like $LINE contains a value which starts with a dash.

You can protect against this with

grep -q -e "$LINE"

More generally, most Unix commands allow -- to mark the end of options, and so any argument after this "end of options" option will be taken as a literal, non-option argument.

echo will have a problem, too; the portable solution is to switch to printf, which works fine with arguments which start with dashes, as long as it's not the first argument, which is a format string.

You should also avoid using uppercase variable names; these are reserved for system use.

Finally, scripts should almost never need to explicitly examine $? - this is already done by if, while and other control constructs.

if grep -q -e "$line" log.log; then
    printf '%s\n' "$line"
fi

As an optimization, placing the redirection outside the loop will make things a lot quicker.

while read -r line; do
    grep -q -e "$line" log.log && printf '%s\n' "$line"
done <results.txt >notfound.txt

Grep keep (output) entire .txt file if one line contains string

For non-GNU versions of grep, which are unlikely to have -z, or if portability is required...

grep -q pattern file && cat file

-q suppresses any output but, per usual, exit status is set based on whether or not a pattern match was found. With a pattern match grep returns the success code 0 which is equivalent to true and that allows the cat command to be executed.

Best Answer

Related Solutions

Shell – grep: Not a recognized flag in AIX

Grep keep (output) entire .txt file if one line contains string

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