The problem of grep command, i want to know how the expression match the result

[xtcsin]

I have a file named a.txt, and its content is as follows:

sdjksdj ksfjs ksj skjf sjf
ls afkfs
sfksj fjskddddddddddjfskdjfsjdkf ksj fks

I run shell command : cat a.txt |grep "fs*"
the result is all the three lines, why?

I run cat a.txt |grep -o "fs*" and the result is as follows:
f
f
f
f
fs
f
f
fs
fs
f
f


I want to know how "fs*" match the result. Thanks.

[aleph]

try the option "--color=auto". You'll see for yourself Usually grep put the whole line containing a match to the output, but the -o switch tells it to print only the matched parts one by one, each on its own separate line.

By the way, don't put patterns in double quotes. You shell will try to expand it before passing it to grep.

For example, if you have a file named "fsA" in the working directory, "fs*" will be expanded as "fsA", and grep will use this pattern (an exact match) instead of the intended "fs*" (f followed by zero or more 's' characters).