Can I use the pipe operator as an OR statement?

Question

Im reading an introductory book on shell commands and it suggests using a pipe command as an OR statement like below:

case "$1" in 
  start|START)
    /usr/bin/sshd
    ;;
  stop|STOP)
    kill $(cat/var/run/sshd.pid)
    ;;
esac

Why does this work/what is the logic behind it?

Answer 1

Here it is not the pipe operator. It is within the case syntax. Read man bash and go to the "Compound Commands" sections and read about the case.

Here is the extract of what the bash manual has to say

case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac

A case command first expands word, and tries to match it against each pattern in turn, using the same matching rules as for pathname expansion (see Pathname Expansion below). The word is expanded using tilde expansion, parameter and variable expansion, arithmetic substitution, command substitution, process substitution and quote removal. Each pattern examined is expanded using tilde expansion, parameter and variable expansion, arithmetic substitution, command substitution, and process substitution. If the shell option nocasematch is enabled, the match is performed without regard to the case of alphabetic characters. When a match is found, the corresponding list is executed. If the ;; operator is used, no subsequent matches are attempted after the first pattern match. Using ;& in place of ;; causes execution to continue with the list associated with the next set of patterns. Using ;;& in place of ;; causes the shell to test the next pattern list in the statement, if any, and execute any associated list on a successful match. The exit status is zero if no pattern matches. Otherwise, it is the exit status of the last command executed in list.

This is the syntax

case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac

Which is

case "$1" in
start|START)

matches the above syntax which tells at-least one pattern, or more than one separated by a | , a vertical bar.