Playing at the Olympic Stadium on the city’s Montjuic hill while their stadium is rebuilt and upgraded over the next year-and-a-half, Barcelona were nearly thwarted by the visitors.
After Getafe held them to a frustrating goalless draw on the opening weekend, Cadiz almost managed the same feat until Ilkay Gundogan cleverly released Pedri to score.
Substitute Torres drove through the middle and tucked home in stoppage time to put gloss on what was another unconvincing performance.
Barca coach Xavi Hernandez, suspended after being sent off against Getafe, selected 16-year-old Lamine Yamal on the right in place of the suspended Raphinha, also dismissed last weekend.
Yamal, on his first start for the club, came closest for the Catalans in the first half with an angled strike expertly tipped to safety by Jeremias Ledesma.
The forward thrilled nearly 40,000 fans — there were around 10,000 empty seats with a low uptake on season tickets because of the temporary move from Camp Nou.
At the other end Barcelona goalkeeper Marc-Andre ter Stegen went one better with a superb save to deny Roger Marti one-on-one, after Alejandro Balde gave the ball away on the halfway line.
New signing Oriol Romeu had a goalbound effort blocked after a scramble in the box early in the second half as Barcelona struggled to break down their Andalucian visitors.
Ruben Alcaraz almost gave the Yellow Submarine the lead but lashed a shot wide, while Chris Ramos also came close with a rasping effort.
Barca midfielder Gavi nodded against the bar at the other end, before Xavi threw on Moroccan winger Ez Abde in his stead.
The coach switched to a three defender system, also bringing on Ansu Fati for Balde as he sought the breakthrough.
Barcelona tossed crosses into the area, looking for Robert Lewandowski, but the Polish forward failed to make a nuisance of himself.
Abde drew a fine save from Ledesma with a vicious curling effort as Barcelona regrouped following a cooling break in the hazy evening heat.
Eventually they found the breakthrough when former Manchester City midfielder Gundogan dinked a cute ball through for Pedri to slide past Ledesma with eight minutes remaining.
The Canary Islander held up a shirt in tribute to Tenerife, currently ravaged by wildfires.
Cadiz almost levelled through Darwin Machis but he hammered a strike over the bar from close range.
Torres secured the victory for Barcelona after he ran on to Lewandowski’s header, through the middle of Cadiz’s crumbling defence and slotted home.
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