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Thanks for the reply. But, I'm still not clear on what s or S is for and how to set them. For example, it's mentioned that If SUID is set, then "x" in the owner permissions is replaced to "s", if owner has execute permissions, or to "S" otherwise.
-rws------ both owner execute and SUID are set
-r-S------ SUID is set, but owner execute is not set
Then, what s or S actually does and how to set s or S?
The setuid bit causes an executable to be run with the permission of the executable's owner, not the permission of the person executing it. it is used to allow a non priviliged user to run a priviliged command such as passwd, which must manipulate the password database. The setgid bit does the same, but for groups. The capital S means that the setuid/setgid bit is set, but not the underlying executable permission (this is extremely rare). A look at man chmod will tell you how to set/unset these bits.