Bash: grep pattern from command output

Question

I'm really new with bash, but it's one of the subjects on school. One of the exercises was:

Give the line number of the file "/etc/passwd" where the information about your own login is.

Suppose USERNAME is my own login ID, I was able to do it perfectly in this way:

 cat /etc/passwd -n | grep USERNAME | cut -f1

Which simply gave the line number required (there may be a more optimised way). I wondered however, if there was a way to make the command more general so that it uses the output of whoami to represent the grep pattern, without scripting or using a variable. In other words, to keep it an easy-to-read one-line command, like so:

 cat /etc/passwd -n | grep (whoami) | cut -f1

Sorry if this is a really noob question.

Solution 1

cat /etc/passwd -n | grep `whoami` | cut -f1 

Surrounding a command in ` marks makes it execute the command and send the output into the command it's wrapped in.

Solution 2

You can do this with a single awk invocation:

awk -v me=$(whoami) -F: '$1==me{print NR}' /etc/passwd

In more detail:

• the -v creates an awk variable called me and populates it with your user name.
• the -F sets the field separator to : as befits the password file.
• the $1==me only selects lines where the first field matches your user name.
• the print outputs the record number (line).

Solution 3

Check command substitution in the bash man page.

You can you back ticks `` or $() , and personally I prefer the latter.

So for your question:

grep -n -e $(whoami) /etc/passwd | cut -f1 -d :

will substitute the output of whoami as the argument for the -e flag of the grep command and the output of the whole command will be line number in /etc/passwd of the running user.

Comments

[MarioDS]

+1 because it works, but Lynn's answer is more clear to read (at least to a beginner like me :)) but I like that you pointed out I should use the -n switch of grep to make it shorter.

[paxdiablo]

+1, I prefer the $() construct as well, simply because it's so much easier to nest.

[MarioDS]

thanks for the great explanation, but for now it seems to advanced for me to build up commands like that myself :)

[paxdiablo]

@Mario: the sooner you start learning, the sooner you'll be able to charge exorbitant rates for your development services :-)

[tripleee]

Without anchoring, this can produce false matches if your login name matches elsewhere on the line. I used to have a short login name on a local system which happened to match (a substring of) "System Operator" so some stupid software thought I was root when I wasn't.