▁expr ▁subst it utes ▁ 1 ▁for ▁$ x ▁while ▁evalu ating ▁the ▁expression ▁puts ▁" The ▁sum ▁of ▁the ▁numbers ▁ 1 .. 5 ▁is ▁$ sum ." ; ▁# ▁sum ▁is ▁ 1 5 ▁ ▁# ▁without ▁cur ly ▁bra ces , ▁variable ▁substitution ▁occurs ▁at ▁the ▁definition ▁site ▁( lex ical ▁scop ing ) ▁set ▁x ▁ 2 ▁set ▁op ▁* ▁set ▁y ▁ 3 ▁set ▁res ▁[ expr ▁$ x $ op $ y ]; ▁# ▁$ x , ▁$ op , ▁and ▁$ y ▁are ▁subst it uted , ▁and ▁the ▁expression ▁is ▁evaluated ▁puts ▁" 2 ▁* ▁ 3 ▁is ▁$ res ." ; ▁# ▁ 6 ▁is ▁subst it uted ▁for ▁$ res ▁ ▁As ▁seen ▁in ▁these ▁examples , ▁there ▁is ▁one ▁basic ▁construct ▁in ▁the ▁language : ▁the ▁command . ▁Qu oting ▁mechan isms ▁and ▁substitution ▁rules ▁determine ▁how ▁the ▁arguments ▁to ▁each ▁command ▁are ▁processed . ▁ ▁One ▁special ▁substitution ▁occurs ▁before ▁the ▁parsing ▁of ▁any ▁commands ▁or ▁arguments . ▁If ▁the ▁final ▁character ▁on ▁a ▁line ▁( i . e ., ▁immediately ▁before ▁a ▁newline ) ▁is ▁a ▁back slash , ▁then ▁the ▁back slash - new line ▁combination ▁( and ▁any ▁spaces ▁or ▁tabs ▁immediately ▁following ▁the ▁newline ) ▁are ▁replaced ▁by ▁a ▁single ▁space . ▁This ▁provides ▁a ▁line ▁continu ation ▁mechanism , ▁where by ▁long ▁lines ▁in ▁the ▁source ▁code ▁can ▁be ▁wrapped ▁to ▁the ▁next ▁line ▁for ▁the ▁convenience ▁of ▁readers . ▁ ▁Cont inu ing ▁with ▁normal