How to use grep to display file names and line numbers before matching lines

grep It's a Linux Utilities , It is usually used to search the contents of a file or any input passed to it . When searching multiple files , It is useful to display the file name and line number , Especially when using it in shell When automating in scripts .

Use grep Show filename

By default , If you pass multiple files to , For the sake of clarity grep, It will show filename: Before matching rows . You can actually turn off this behavior with flags , The -h The flag never shows the file name ：

however , If you only pass one file to grep, By default, it will not display the file name . In the use of shell When scripts are automated , This could be a problem , Because you may not know how many files are in the directory , And it may break the automation that depends on file names .

The simple solution is to use uppercase -H sign , It is the opposite of the file name ,-h And the file name will be opened anyway , Even if only one file is passed as input .

grep -H "foo" file

The -H The logo has another unexpected but useful effect —— When and from The input of （ for example Unix The Conduit ） When pairing stdin, It will (standard input): Print instead of file name .

Use grep According to the line Numbers

You can also associate it with -n Flags are used together to get line numbers ：

grep -Hn "foo"

accord with POSIX

Medium -H sign grep Do not conform to the POSIX, And in some more obscure ones based on Unix Is not available in the operating system of . Fortunately, , You can use a hack, By way of /dev/null The forged second file input is passed to grep, This will trick it into thinking that there are multiple files ：