CODEWITHSUNDEEP
bashshellgrep
Bogdan Molinger
Bogdan Molinger

Reputation: 341

unusual chaining of "grep" in the shell

I stumbled upon a shell instruction which looks odd:
ls -a | grep ".qmail-" | grep -v "mail" | grep ".mail" > t ; echo $?
I suspect that the returned value would represent an error. Could anyone confirm this or explain in which circumstances this instruction would be applied ?

Upvotes: 1

Views: 247

Answers (2)

Adrian Stoll
Adrian Stoll

Reputation: 31

The bash shell (and most other shells) let users use the output of one command as the input of another. This is accomplished with the | operator which is called a pipe. So the output of of ls -a is fed to grep ".qmail-" and so on. The > operator sends the output of the command to a file, in this case t. So ls -a | grep ".qmail-" | grep -v "mail" | grep ".mail" > t lists the contents of a directory and passes the output through successive filters before finally saving the output to the file t.

The semicolon signals the end of a command and allows multiple bash commands to be entered on a single line.

echo $? prints out the return value of the last executed command, in this case, ls -a | grep ".qmail-" | grep -v "mail" | grep ".mail" > t. By convention, any value besides 0 indicates some sort of error occurred.The Linux Documentation Project gives a list of some common exit codes.

Upvotes: 1

Jonathan Leffler
Jonathan Leffler

Reputation: 754550

The first grep only allows through lines that contain qmail (preceded by any character and followed by a dash, but that is largely immaterial). The second grep strips out lines that contain mail, which means every line passed by the first grep is deleted by the second. There's nothing left for the third one to process, so the file t will always be empty. The value for $? should be 1, failure, since the third grep failed to find any lines that matched its pattern (because it got no lines to process).

It is a mistake.

It is hard to know how to fix it without knowing what it is trying to do.

Upvotes: 4

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